C++ Program To Reverse Words In A Given String

15 Dec, 2021
Example: Let the input string be “i like this program very much”. The function should change the string to “much very program this like i”
Examples: 
Input: s = “geeks quiz practice code” Output: s = “code practice quiz geeks”
Input: s = “getting good at coding needs a lot of practice” Output: s = “practice of lot a needs coding at good getting”
Algorithm:  
Initially, reverse the individual words of the given string one by one, for the above example, after reversing individual words the string should be “i ekil siht margorp yrev hcum”.
Reverse the whole string from start to end to get the desired output “much very program this like i” in the above example.
Below is the implementation of the above approach: 
C++
// C++ program to reverse a string#include <bits/stdc++.h>using namespace std;  // Function to reverse wordsvoid reverseWords(string s){        // Temporary vector to store all words    vector<string> tmp;    string str = "";    for (int i = 0; i < s.length(); i++)     {                // Check if we encounter space         // push word(str) to vector        // and make str NULL        if (s[i] == ' ')         {            tmp.push_back(str);            str = "";        }          // Else add character to         // str to form current word        else            str += s[i];    }        // Last word remaining,add it to vector    tmp.push_back(str);      // Now print from last to first in vector    int i;    for (i = tmp.size() - 1; i > 0; i--)        cout << tmp[i] << " ";    // Last word remaining,print it    cout << tmp[0] << endl;}  // Driver Codeint main(){    string s =     "i like this program very much";    reverseWords(s);    return 0;}
Output:
much very program this like i
The above code doesn’t handle the cases when the string starts with space. The following version handles this specific case and doesn’t make unnecessary calls to reverse function in the case of multiple spaces in between. Thanks to rka143 for providing this version. 
C++
// C++ program to implement// the above approachvoid reverseWords(char* s){  char* word_begin = NULL;    // temp is for word boundary   char* temp = s;    // STEP 1 of the above algorithm   while (*temp)   {    /*This condition is to make sure that       the string start with valid character       (not space) only*/    if ((word_begin == NULL) &&         (*temp != ' '))     {      word_begin = temp;    }    if (word_begin &&        ((*(temp + 1) == ' ') ||       (*(temp + 1) == '')))     {      reverse(word_begin, temp);      word_begin = NULL;    }    temp++;    // End of while   }     // STEP 2 of the above algorithm   reverse(s, temp - 1);}  // This code is contributed by rutvik_56.
Time Complexity: O(n) 
Another Approach:
we can do the above task by splitting and saving the string in a reverse manner. 
Below is the implementation of the above approach:
C++
// C++ program to reverse a string// s = input()#include <bits/stdc++.h>using namespace std;  // Driver codeint main(){    string s[] = {"i", "like", "this",                   "program", "very", "much"};                                              string ans = "";    for (int i = 5; i >= 0; i--)     {        ans += s[i] + " ";    }    cout <<     ("Reversed String:") << endl;    cout <<     (ans.substr(0, ans.length() - 1)) <<      endl;      return 0;}// This code is contributed by divyeshrabadiya07.
Output:
Reversed String:
much very program this like i
Time Complexity: O(n) 
Without using any extra space:The above task can also be accomplished by splitting and directly swapping the string starting from the middle. As direct swapping is involved, less space is consumed too.   
Below is the implementation of the above approach:
C++
// C++ code to reverse a string#include <bits/stdc++.h>using namespace std;  // Reverse the stringstring RevString(string s[], int l){        // Check if number of words is even    if (l % 2 == 0)    {                // Find the middle word        int j = l / 2;                  // Starting from the middle        // start swapping words at         // jth position and l-1-j position        while (j <= l - 1)        {            string temp;            temp = s[l - j - 1];            s[l - j - 1] = s[j];            s[j] = temp;            j += 1;        }    }          // Check if number of words is odd    else    {                // Find the middle word        int j = (l / 2) + 1;                  // Starting from the middle start        // swapping the words at jth         // position and l-1-j position        while (j <= l - 1)         {            string temp;            temp = s[l - j - 1];            s[l - j - 1] = s[j];            s[j] = temp;            j += 1;        }    }          string S = s[0];          // Return the reversed sentence    for(int i = 1; i < 9; i++)    {        S = S + " " + s[i];    }    return S;}  // Driver codeint main(){    string s = "getting good at coding "               "needs a lot of practice";                     string words[] = {"getting", "good", "at",                       "coding", "needs", "a",                       "lot", "of", "practice"};           cout << RevString(words, 9) << endl;    return 0;}  // This code is contributed by divyesh072019
Output:
practice of lot a needs coding at good getting
Another intuitive constant space solution: Go through the string and mirror each word in the string, then, at the end, mirror the whole string.
The following C++ code can handle multiple contiguous spaces.
C++
// C++ program to implement// the above approach#include <algorithm>#include <iostream>#include <string> using namespace std;   string reverse_words(string s){    int left = 0, i = 0, n = s.size();         while (s[i] == ' ')        i++;         left = i;         while (i < n)    {        if (i + 1 == n ||             s[i] == ' ')        {            int j = i - 1;            if (i + 1 == n)                j++;                         while (left < j)                swap(s[left++], s[j--]);                         left = i + 1;        }        if (s[left] == ' ' && i > left)            left = i;                 i++;    }    reverse(s.begin(), s.end());    return s;}  // Driver codeint main(){     string str =     "Be a game changer the world is already full of players";     str = reverse_words(str);     cout << str;       return 0;}// This code is contributed by Gatea David
Output:
players of full already is world the changer game a Be
Please refer complete article on Reverse words in a given string for more details!
Accolite
Adobe
Amazon
CBSE - Class 11
Cisco
Goldman Sachs
MakeMyTrip
MAQ Software
Microsoft
Morgan Stanley
Paytm
Payu
Reverse
SAP Labs
school-programming
Wipro
Zoho
C++ Programs
Strings
Paytm
Zoho
Morgan Stanley
Accolite
Amazon
Microsoft
MakeMyTrip
Payu
Goldman Sachs
MAQ Software
Adobe
Wipro
SAP Labs
Cisco
Strings
Reverse
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Passing a function as a parameter in C++
Const keyword in C++
Program to implement Singly Linked List in C++ using class
cout in C++
Different ways to print elements of vector
Write a program to reverse an array or string
Reverse a string in Java
Write a program to print all permutations of a given string
C++ Data Types
Check for Balanced Brackets in an expression (well-formedness) using Stack