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”

reverse-words

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:

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// CPP 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);
    getchar();
    return 0;
}

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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 space in between. Thanks to rka143 for providing this version.

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void reverseWords(char* s)
{
    char* word_begin = NULL;
    char* temp = s; /* temp is for word boundry */
  
    /*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) == '\0'))) {
            reverse(word_begin, temp);
            word_begin = NULL;
        }
        temp++;
    } /* End of while */
  
    /*STEP 2 of the above algorithm */
    reverse(s, temp - 1);
}

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Time Complexity: O(n)
we can do the above task by splitting and saving the string in reverse manner.

Java

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// Java program to reverse a string
// s = input()
public class ReverseWords {
  
    public static void main(String[] args)
    {
        String s[] = "i like this program very much".split(" ");
        String ans = "";
        for (int i = s.length - 1; i >= 0; i--) {
            ans += s[i] + " ";
        }
        System.out.println("Reversed String:");
        System.out.println(ans.substring(0, ans.length() - 1));
    }
}

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C#

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// C# program to reverse a string
// s = input()
  
using System;
public class ReverseWords {
  
    public static void Main()
    {
        string[] s = "i like this program very much".Split(' ');
        string ans = "";
        for (int i = s.Length - 1; i >= 0; i--) {
            ans += s[i] + " ";
        }
        Console.Write("Reversed String:\n");
        Console.Write(ans.Substring(0, ans.Length - 1));
    }
}

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Python3

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# Python3 program to reverse a string
# s = input()
s = "i like this program very much"
words = s.split(' ')
string =[]
for word in words:
    string.insert(0, word)
  
print("Reversed String:")
print(" ".join(string))
  
# Solution proposed bu Uttam

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Output:

Reversed String:
much very program this like i

Please write comments if you find any bug in above code/algorithm, or find other ways to solve the same problem.



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