sabotage-1

See Russian Translation

ZYX is a famous international-level linguistics and informatics competitor. He is the favorite to win this year's IOI competition.

Fifiman, another great coder in his own right, known as the "King of Algorithms", was unfortunately overshadowed by ZYX's outstanding ability many times in various informatics competitions. To this day, he is burning with bitter regret and loathing towards the young genius.

This year's IOI is held in country KZ. Naturally, when Fifiman heard that ZYX would be travelling to country KZ directly from the country where ZYX's linguistics olympiad is taking place, he had a malicious thought. If Fifiman can make the journey as painful as possible, perhaps ZYX will be too tired to win the IOI! Since Fifiman is also an accomplished hacker, he decided to break into the systems of some airline companies to cancel some flights.

For simplicity, you can assume that a flight is a direct two-way flight between two countries. ZYX is going to travel from the country where the linguistics olympiad is being held to country KZ, possibly switching flights several times. An itinerary, or sequence of flights, is considered valid if and only if it does not visit the same country more than once. Additionally, each two consecutive cities in the itinerary should be connected by a direct flight.

Unfortunately, Fifiman is not sure where the linguistics olympiad is being held. He also does not know exactly which country is country KZ. Therefore, he has decided to cancel flights in such a way that ZYX's journey, no matter which two countries he is travelling between, is inconvenient. Travelling between two countries is considered inconvenient if and only if there is exactly one valid itinerary. Note that it should still be possible to travel between any two countries by this definition.

Among all possible ways to cancel flights, Fifiman is interested in the way that cancels the most flights. If there are multiple ways to do so, Fifiman will take the lexicographically minimal solution, and he will cancel the flights with the smaller indices before the flights with the larger indices. And it turns out that you work for one of the airline companies Fifiman is going to hack. To prevent a major communications breakdown, you first need to figure out which flights Fifiman is planning to cancel!

Note: To compare two different solutions, find the first cancelled flight that is different in the order Fifiman cancels the flights. The solution with the smaller indexed flight at that position is considered to be lexicographically smaller than the other solution. The smallest solution out of all the possible solutions is considered to be the lexicographically minimal solution.

Input Format: 
The first line of input will contain N and M, the number of countries and the number of direct flights, respectively. 
The next M lines of input will each contain two integers u and v, indicating that there is a direct flight between countries u and v. There may be multiple direct flights between any pair of countries, and no country will have a flight to itself. The flights are numbered from 1 to M in the order they are given in the input.

Output Format: 
The first line of output should contain an integer K, the number of flights Fifiman will cancel. 
The next K lines should contain the indices of flights Fifiman cancels (according to the problem statement, these will be in ascending order. It is guaranteed that a solution always exists.

Constraints: 
For all subtasks, 
1 ≤ u, v ≤ N 
u ≠ v 
[Subtask 1 - 20%] 
2 ≤ N ≤ 20 
1 ≤ M ≤ 190 
[Subtask 2 - 40%] 
2 ≤ N ≤ 1000 
1 ≤ M ≤ 1000 
[Subtask 3 - 40%] 
2 ≤ N ≤ 10^5 
1 ≤ M ≤ 10^5 

SAMPLE INPUT
3 4
2 1
1 2
2 3
3 1

SAMPLE OUTPUT
2
1
2

Explanation

First, Fifiman should cancel the first flight, as it's a duplicate of the second flight. Then, Fifiman could cancel any of the three remaining flights, but out of the three choices, cancelling the first remaining one is the lexicographically minimal solution.

Difficulty: MEDIUM
Tags: Spanning trees

Data structures

Greedy algorithms
Skill types: Data structures

Greedy algorithms

Starter code:

Solutions:
N,M=list(map(int,input().split()))
print(M-N+1)
weights=[1]*(N+1)
roots=[i for i in range(N+1)]
def root(n):
	while roots[n]!=n:
		n=roots[n]
	return n
def merge(i,j):
	a=root(i)
	b=root(j)
	#print "merging",a,b
	if a==b: return 0
	if weights[a]<weights[b]:
		a,b=b,a
	roots[b]=a
	weights[a]+=weights[b]
	return 1

f=[]
for i in range(M):
	a,b=list(map(int,input().split()))
	f.append((a,b))
res=[]
i=M
for a,b in reversed(f):
	if not merge(a,b):
		res.append(i)
	i-=1	
print("\n".join(map(str,reversed(res))))