C++ Program To Reverse Words In A Given String

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 words
void 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 Code
int 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 approach
void 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 code
int 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 string
string 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 code
int 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 code
int 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!