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 <stdio.h>

// Function to reverse any sequence
// starting with pointer begin and
// ending with pointer end
void reverse(char* begin,char* end)
{
    char temp;
    while (begin < end)
    {
        temp = *begin;
        *begin++ = *end;
        *end-- = temp;
    }
}

// Function to reverse words
void reverseWords(char* s)
{
    char* word_begin = s;

    // Word boundary
    char* temp = s;

    // Reversing individual words as
    // explained in the first step
    while (*temp)
    {
        temp++;
        if (*temp =='\0')
        {
            reverse(word_begin,temp - 1);
        }
        else if (*temp == ' ')
        {
            reverse(word_begin,temp - 1);
            word_begin = temp + 1;
        }
    }

    // Reverse the entire string
    reverse(s, temp - 1);
}

// Driver Code
int main()
{
    char s[] =
    "i like this program very much";
    char* temp = s;
    reverseWords(s);
    printf("%s", s);
    return 0;
}

Output

much very program this like i

Time Complexity: O(n)
Auxiliary Space: O(n) 

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);
}

Time Complexity: O(n) 
Another Approach:

Please refer complete article on Reverse words in a given string for more details!