Travelling Salesman Problem Using Dynamic Programming
Tags: programming, opensource, coding, career
Published: 2023-09-10 19:47:57

The Travelling Salesman Problem (TSP) is a well-known problem in computer science and operations research. It involves finding the shortest possible route that allows a salesman to visit a given set of cities and return to their starting point.

In more formal terms, the TSP can be defined as follows: given a set of cities, a distance function that measures the distance between any two cities, and a starting city, the goal is to find a path that visits each city exactly once and returns to the starting city, while minimizing the total distance traveled.

The TSP is a classic example of an NP-hard problem, meaning that it is extremely difficult to solve and there is no known algorithm that can guarantee an optimal solution in polynomial time. As a result, various heuristics and approximate algorithms have been developed to find good, but not necessarily optimal, solutions to the problem.

Real-world applications of the TSP include optimizing delivery routes for companies, scheduling visits for salespeople, and planning tours for tourists. The TSP is also used as a benchmark problem to test the performance of various optimization algorithms. In this article we will learn a step-by-step example of how to use dynamic programming to solve the [Travelling Salesman Problem (TSP) using Dynamic Programming ](https://www.interviewbit.com/blog/travelling-salesman-problem/)

**Examples of real-world applications?**

There are many real-world applications of the Travelling Salesman Problem (TSP). Some examples include:

1. Delivery routes: Companies that deliver goods often need to find the most efficient routes for their vehicles. The TSP can be used to optimize these routes by finding the shortest possible path that allows the vehicle to visit all of the delivery locations and return to its starting point.
2. Sales routes: Salespeople often need to visit multiple locations in a given region. The TSP can be used to help them plan their visits in the most efficient way, by finding the shortest route that allows them to visit all of their potential customers.
3. Tour planning: Tour operators and travel agencies can use the TSP to plan the most efficient routes for their tours, taking into account the distances between different destinations and the amount of time available for the tour.
4. Logistics: The TSP can be used to optimize logistics operations, such as warehouse management and supply chain management, by finding the most efficient routes for transporting goods between different locations.
5. Benchmarking: The TSP is often used as a benchmark problem to test the performance of various optimization algorithms. This helps researchers and practitioners to compare the effectiveness of different approaches to solving the problem.

**Overview of different approaches to solving the problem?**
There are several approaches that can be used to solve the Travelling Salesman Problem (TSP). Some of the most common approaches include:

1. Exact algorithms: These algorithms aim to find the exact, optimal solution to the TSP. Examples of exact algorithms include branch and bound, dynamic programming, and integer linear programming. These approaches can be very effective, but they may not scale well to large instances of the TSP due to the computational complexity of the problem.
2. Heuristics: Heuristics are approximate algorithms that aim to find good, but not necessarily optimal, solutions to the TSP. Examples of heuristics include nearest neighbor, simulated annealing, and genetic algorithms. These approaches can be faster than exact algorithms, but the solutions they produce may not be as good.
3. [Brute force](https://www.imperva.com/learn/application-security/brute-force-attack/#:~:text=A%20brute%20force%20attack%20involves,scripts%20as%20brute%20force%20tools.): The brute force approach involves generating and testing all possible routes and selecting the shortest one. This approach is guaranteed to find the optimal solution, but it is only practical for small instances of the TSP due to the exponential time complexity.
4. Approximation algorithms: These algorithms aim to find solutions that are within a certain distance of the optimal solution. They can be faster than exact algorithms, but the solutions they produce may not be as good.
5. Metaheuristics: Metaheuristics are high-level strategies that can be used to guide the search for a solution to the TSP. They often involve the use of heuristics or approximation algorithms, but they may also incorporate other techniques such as local search or population-based optimization.

Each of these approaches has its own strengths and limitations, and the best approach for a particular instance of the TSP will depend on the specific needs and constraints of the problem.

**Definition and explanation of dynamic programming?**
Dynamic programming is a method of solving optimization problems by breaking them down into smaller subproblems, solving each subproblem independently, and storing the solutions to the subproblems in a table or array. When a subproblem is encountered again, its solution can be looked up in the table instead of being recomputed, which can greatly improve the efficiency of the overall solution.

Dynamic programming is particularly well-suited for solving problems that have an optimal substructure, meaning that the optimal solution to the problem can be constructed from the optimal solutions to its subproblems. It is also often used for problems that have overlapping subproblems, which means that the same subproblems are encountered multiple times during the course of the computation.

To use dynamic programming to solve a problem, it is often necessary to first identify a recursive structure for the problem and then apply the following steps:

1. Define a recursive function that takes as input the current state of the problem and returns the optimal solution for that state.
2. Define a table or array to store the solutions to the subproblems.
3. Use the recursive function to compute the solutions to the subproblems and store them in the table.
4. Use the stored solutions to the subproblems to compute the solution to the original problem.

[Dynamic programming](https://www.spiceworks.com/tech/devops/articles/what-is-dynamic-programming/#:~:text=Dynamic%20programming%20is%20defined%20as,minimum%20range%20of%20the%20algorithmic) can be a powerful tool for solving optimization problems, but it can also be computationally intensive and may not be the most efficient approach for all problems.

**How dynamic programming can be applied to the Travelling Salesman Problem?**

Dynamic programming can be applied to the Travelling Salesman Problem (TSP) by using it to solve the following subproblem: given a set of cities and a starting city, what is the shortest possible route that visits all of the cities and returns to the starting city?

To solve this subproblem using dynamic programming, we can define a recursive function that takes as input the current state of the problem (i.e., the set of cities that have not yet been visited and the current city) and returns the optimal solution for that state (i.e., the shortest possible route that visits all of the remaining cities and returns to the starting city).

We can then use this recursive function to compute the solutions to the subproblems and store them in a table or array. This table can be used to look up the solutions to the subproblems when they are encountered again, which can greatly improve the efficiency of the overall solution.

Finally, we can use the stored solutions to the subproblems to compute the solution to the original TSP. The overall solution will be the shortest possible route that visits all of the cities and returns to the starting city.
One important consideration when using dynamic programming to solve the TSP is the order in which the subproblems are solved. It is typically most efficient to solve the subproblems in a specific order, such as by starting with the smallest subproblems and working up to the largest ones. This can help to reduce the number of subproblems that need to be solved and can make the solution more efficient.

**Advantages and disadvantages of using dynamic programming for this problem?**
Using dynamic programming to solve the Travelling Salesman Problem (TSP) has several advantages:

1. Guaranteed optimality: Dynamic programming is an exact algorithm, which means that it is guaranteed to find the optimal solution to the TSP (assuming that the problem is defined correctly and that the recursive function is implemented correctly).
2. Efficient solution: Dynamic programming can be very efficient, especially for small instances of the TSP. By storing the solutions to the subproblems in a table, we can avoid the need to recompute these solutions, which can greatly improve the overall efficiency of the solution.
3. Simple implementation: Dynamic programming is a relatively simple and straightforward approach to solving the TSP. It is often easier to implement than other exact algorithms, such as branch and bound.

However, dynamic programming also has some disadvantages:
High computational complexity: The TSP is an NP-hard problem, which means that it is extremely difficult to solve and there is no known algorithm that can guarantee an optimal solution in polynomial time. As a result, dynamic programming can be computationally intensive and may not scale well to large instances of the TSP.
Limited to problems with an optimal substructure: Dynamic programming can only be used to solve problems that have an optimal substructure, meaning that the optimal solution to the problem can be constructed from the optimal solutions to its subproblems. This may not be the case for all instances of the TSP.
May not be the most efficient approach: While dynamic programming can be efficient for small instances of the TSP, it may not be the most efficient approach for larger instances or for problems with special characteristics. In these cases, other approaches, such as heuristics or approximation algorithms, may be more suitable.

**Step-by-step example of solving the problem using dynamic programming?** :

1. Define the problem: The first step is to define the TSP in terms of a set of cities, a distance function that measures the distance between any two cities, and a starting city. We will assume that the distance function is symmetric, which means that the distance from city A to city B is the same as the distance from city B to city A.
2. Define a recursive function: Next, we will define a recursive function that takes as input the current state of the problem (i.e., the set of cities that have not yet been visited and the current city) and returns the shortest possible route that visits all of the remaining cities and returns to the starting city. This function will be used to compute the solutions to the subproblems.
3. Define a table to store the solutions to the subproblems: We will define a table or array to store the solutions to the subproblems. This table will be used to look up the solutions to the subproblems when they are encountered again, which can greatly improve the efficiency of the overall solution.
4. Initialize the table: We will initialize the table by setting the value of the starting city to 0. This represents the fact that it costs 0 distance to travel from the starting city to itself.
5. Compute the solutions to the subproblems: We will use the recursive function to compute the solutions to the subproblems and store them in the table. To do this, we will iterate over the subproblems in a specific order (e.g., starting with the smallest subproblems and working up to the largest ones), and use the stored solutions to the subproblems to compute the solutions to the current subproblem.
6. Use the stored solutions to compute the solution to the original TSP: Finally, we will use the stored solutions to the subproblems to compute the solution to the original TSP. The overall solution will be the shortest possible route that visits all of the cities and returns to the starting city.

Note that this is just one possible way to solve the TSP using dynamic programming, and the specific details of the algorithm will depend on the specific needs and constraints of the problem.

Complete code using dynamic programming and Detailed explanation of the code and algorithm used: Here is an example of how to use dynamic programming to solve the Travelling Salesman Problem (TSP) in Python:

```
def tsp(cities, start, distance_table, cache):
  # If the current state is in the cache, return the stored solution
  if cache.get(start) is not None:
    return cache[start]
  
  # Base case: if there is only one city left to visit, return the distance
  # to that city and the path to that city
  if len(cities) == 1:
    return distance_table[start][cities[0]], [start, cities[0]]
```
  
  

```
# Initialize the minimum distance and path to infinity and an empty list
  min_distance = float('inf')
  min_path = []
  
  # Iterate over the remaining cities
  for next_city in cities:
    # Calculate the distance from the current city to the next city
    distance = distance_table[start][next_city]
    
    # Recursively compute the shortest possible route that visits all of the
    # remaining cities and returns to the starting city, starting at the next city
    remaining_cities = [city for city in cities if city != next_city]
    sub_distance, sub_path = tsp(remaining_cities, next_city, distance_table, cache)
    
    # Update the minimum distance and path if the current solution is better
    if distance + sub_distance < min_distance:
      min_distance = distance + sub_distance
      min_path = [start] + sub_path
  
  # Store the minimum distance and path in the cache and return the result
  cache[start] = min_distance, min_path
  return min_distance, min_path 

`# Example usage
cities = ['A', 'B', 'C', 'D']
distance_table = {
  'A': {'A': 0, 'B': 5, 'C': 3, 'D': 4},
  'B': {'A': 5, 'B': 0, 'C': 1, 'D': 2},
  'C': {'A': 3, 'B': 1, 'C': 0, 'D': 5},
  'D': {'A': 4, 'B': 2, 'C': 5, 'D': 0}
}
start = 'A'
cache = {}

min_distance, min_path = tsp(cities, start, distance_table, cache)
print(f'Minimum distance: {min_distance}')
print(f'Minimum path: {min_path}')
```

This code defines a recursive function tsp() that takes as input the set of cities, the starting city, a distance table that stores the distances between the cities, and a cache to store the solutions to the subproblems. The function returns the minimum distance and path that visits all of the cities and returns to the starting city.

The function first checks if the current state (i.e., the set of cities that have not yet been visited and the current city) is in the cache. If it is, the stored solution is returned. This helps to avoid the need to recompute the solution to the same subproblem multiple times.

The function then checks if there is only one city left to visit. If this is the case, the distance to that city is returned along with the path to that city. This is the base case of the recursive function.

If there are more cities left to visit, the function initializes the minimum distance and path to infinity and an empty list. It then iterates over the remaining cities and, for each city, calculates the distance from the current city to the next city. It then recursively calls itself to compute the shortest possible route that visits all of the remaining cities and returns to the starting city, starting at the next city.

Finally, the function updates the minimum distance and path if the current solution is better. It then stores the minimum distance and path in the cache and returns the result.

To use this code to solve the TSP, we simply need to define the set of cities, the starting city, and the distance table, and then call the tsp() function with these inputs. The function will return the minimum distance and path that visits all of the cities and returns to the starting city.

In this example, the distance table is represented as a dictionary of dictionaries, where the keys are the cities and the values are dictionaries that map each city to its distance from the current city. The tsp() function uses this distance table to calculate the distances between the cities.

The tsp() function uses a bottom-up approach to solve the TSP, starting with the smallest subproblems and working up to the largest ones. This can be a very efficient approach, especially for small instances of the TSP, but it may not scale well to larger instances due to the high computational complexity of the problem.

**Detailed explanation of the code and algorithm used?**

The code provided in the previous response defines a recursive function tsp() that uses dynamic programming to solve the Travelling Salesman Problem (TSP). The function takes as input the set of cities, the starting city, a distance table that stores the distances between the cities, and a cache to store the solutions to the subproblems. It returns the minimum distance and path that visits all of the cities and returns to the starting city.

The function first checks if the current state (i.e., the set of cities that have not yet been visited and the current city) is in the cache. If it is, the stored solution is returned. This helps to avoid the need to recompute the solution to the same subproblem multiple times, which can greatly improve the efficiency of the overall solution.

If the current state is not in the cache, the function checks if there is only one city left to visit. If this is the case, the distance to that city is returned along with the path to that city. This is the base case of the recursive function.

If there are more cities left to visit, the function initializes the minimum distance and path to infinity and an empty list. It then iterates over the remaining cities and, for each city, calculates the distance from the current city to the next city. It then recursively calls itself to compute the shortest possible route that visits all of the remaining cities and returns to the starting city, starting at the next city.

The recursive function is called with a modified version of the input cities, where the next city has been removed from the list. This helps to ensure that each city is visited exactly once.
After the recursive call, the function updates the minimum distance and path if the current solution is better. It then stores the minimum distance and path in the cache and returns the result.

To use this code to solve the TSP, we simply need to define the set of cities, the starting city, and the distance table, and then call the tsp() function with these inputs. The function will return the minimum distance and path that visits all of the cities and returns to the starting city.

The tsp() function uses a bottom-up approach to solve the TSP, starting with the smallest subproblems and working up to the largest ones. This can be a very efficient approach, especially for small instances of the TSP, but it may not scale well to larger instances due to the high computational complexity of the problem.

In this example, the distance table is represented as a dictionary of dictionaries, where the keys are the cities and the values are dictionaries that map each city to its distance from the current city. The tsp() function uses this distance table to calculate the distances between the cities.

The tsp() function uses a recursive structure to solve the TSP. It breaks the problem down into smaller subproblems and solves each subproblem independently. It then uses the solutions to the subproblems to compute the solution to the original problem. This approach can be very effective for solving optimization problems, but it can also be computationally intensive and may not scale well to large instances of the TSP.

**Tips and tricks for optimizing the solution?** Here are some tips and tricks for optimizing the solution to the Travelling Salesman Problem (TSP) using dynamic programming:

1. Use a good data structure to store the solutions to the subproblems: A good data structure can make a big difference in the efficiency of the overall solution. For example, using a hash table or dictionary to store the solutions to the subproblems can be more efficient than using a list or array, especially for large instances of the TSP.
2. Choose an efficient order for solving the subproblems: The order in which the subproblems are solved can have a big impact on the efficiency of the overall solution. It is typically most efficient to solve the subproblems in a specific order, such as by starting with the smallest subproblems and working up to the largest ones.
3. Use memoization to avoid recomputing the solutions to the same subproblems: Memoization is a technique that involves storing the solutions to the subproblems in a cache so that they can be looked up instead of being recomputed when they are encountered again. This can greatly improve the efficiency of the solution.
4. Use an approximation algorithm or heuristic for large instances of the TSP: For large instances of the TSP, dynamic programming may not be the most efficient approach. In these cases, it may be more effective to use an approximation algorithm or heuristic, which can provide a good solution in a shorter amount of time but may not guarantee an optimal solution.
5. Use parallelization to speed up the computation: If you have access to multiple processors or cores, you can use parallelization to speed up the computation by dividing the subproblems among the processors and solving them concurrently. This can significantly reduce the overall runtime of the solution.
6. Use a better distance function: The distance function used to calculate the distances between the cities can have a big impact on the efficiency of the solution. Using a more efficient distance function, such as the Euclidean distance, can significantly improve the efficiency of the solution.

**Alternative Approaches to Solving the Travelling Salesman Problem:** 

Overview of other approaches, such as brute force and heuristics?
In addition to dynamic programming, there are several other approaches that can be used to solve the Travelling Salesman Problem (TSP):

1. Brute force: One approach to solving the TSP is to simply try all possible routes and return the shortest one. This is known as a brute force approach, and it is guaranteed to find the optimal solution if it is run to completion. However, the computational complexity of this approach is exponential, which means that it may not be practical for large instances of the TSP.
2. [Heuristics](https://en.wikipedia.org/wiki/Heuristic): Another approach to solving the TSP is to use heuristics, which are algorithms that aim to find a good solution to the problem in a reasonable amount of time, but do not guarantee an optimal solution. Examples of heuristics for the TSP include the nearest neighbor algorithm, the 2-opt algorithm, and the simulated annealing algorithm.
3. Approximation algorithms: Approximation algorithms are algorithms that aim to find a solution to the TSP that is close to the optimal solution, but may not be exactly the same as the optimal solution. These algorithms can be much faster than exact algorithms, but they may not provide the same level of accuracy. Examples of approximation algorithms for the TSP include the Christofides algorithm and the Held-Karp algorithm.
4. Metaheuristics: Metaheuristics are algorithms that use heuristics and other techniques to find a good solution to the TSP in a reasonable amount of time. These algorithms can be very effective for solving large and complex optimization problems, but they may not guarantee an optimal solution. Examples of metaheuristics for the TSP include the genetic algorithm and the ant colony optimization algorithm.
5. Branch and bound: Branch and bound is a technique that can be used to solve optimization problems, including the TSP, by systematically exploring the space of possible solutions and eliminating suboptimal solutions. This approach can be very effective for solving the TSP, but it can be computationally intensive and may not scale well to large instances of the problem.
6. Linear programming: Linear programming is a technique that can be used to solve optimization problems with linear constraints. By expressing the TSP as a linear program, it is possible to use linear programming techniques to find a good solution to the problem.
7. Constraint programming: Constraint programming is a technique that can be used to solve optimization problems with complex constraints. By expressing the TSP as a constraint program, it is possible to use constraint programming techniques to find a good solution to the problem.

Overall, the choice of approach to solving the TSP will depend on the specific needs and constraints of the problem, including the size of the problem, the level of accuracy required, and the available computational resources.

Comparison table of these approaches to dynamic programming in terms of efficiency and effectiveness one by one: Here is a comparison of the different approaches to solving the Travelling Salesman Problem (TSP) in terms of efficiency and effectiveness:


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/o2bukxpfb04vsun2hb69.jpeg)

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/sqi8e1vv0bj89ohbt3hg.jpeg)

- Brute force: Brute force is a straightforward approach that guarantees to find the optimal solution if it is run to completion, but it has a very high computational complexity and may not be practical for large instances of the TSP.
- Heuristics: Heuristics are algorithms that aim to find a good solution to the problem in a reasonable amount of time, but do not guarantee an optimal solution. They can be relatively efficient, but their effectiveness can vary depending on the specific heuristic being used.
- [Approximation algorithms](https://en.wikipedia.org/wiki/Approximation_algorithm): Approximation algorithms are algorithms that aim to find a solution to the TSP that is close to the optimal solution, but may not be exactly the same as the optimal solution. These algorithms can be very efficient, but they may not provide the same level of accuracy as exact algorithms.
- Metaheuristics: Metaheuristics are algorithms that use heuristics and other techniques to find a good solution to the TSP in a reasonable amount of time. These algorithms can be very efficient, but they may not guarantee an optimal solution.
- Branch and bound: Branch and bound is a technique that can be very effective for solving the TSP, but it can be computationally intensive and may not scale well to large instances of the problem.
- Linear programming: Linear programming is a technique that can be very effective for solving the TSP, but it requires that the problem can be expressed as a linear program, which may not always be possible

**Conclusion**:

In this blog, we have discussed the Travelling Salesman Problem (TSP) and several different approaches that can be used to solve it, including dynamic programming, brute force, heuristics, approximation algorithms, metaheuristics, branch and bound, linear programming, and constraint programming.

In conclusion, the Travelling Salesman Problem is a fascinating and challenging problem that has a wide range of real-world applications. There are many different approaches that can be used to solve it, each with its own strengths and weaknesses. By understanding the different approaches and their trade-offs, it is possible to choose the best approach for a given problem and to continue improving upon it in the future.