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Program to Reverse a String using Pointers

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  • Difficulty Level : Medium
  • Last Updated : 19 Jul, 2022
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Given a string, the task is to reverse this String using pointers. Examples:

Input: Geeks
Output: skeeG

Input: GeeksForGeeks
Output: skeeGroFskeeG

Approach: This method involves taking two pointers, one that points at the start of the string and the other at the end of the string. The characters are then reversed one by one with the help of these two pointers. Program: 

C++




#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse the string
// using pointers
void reverseString(char* str)
{
  int l, i;
  char *begin_ptr, *end_ptr, ch;
 
  // Get the length of the string
  l = strlen(str);
 
  // Set the begin_ptr and end_ptr
  // initially to start of string
  begin_ptr = str;
  end_ptr = str;
 
  // Move the end_ptr to the last character
  for (i = 0; i < l - 1; i++)
    end_ptr++;
 
  // Swap the char from start and end
  // index using begin_ptr and end_ptr
  for (i = 0; i < l / 2; i++) {
 
    // swap character
    ch = *end_ptr;
    *end_ptr = *begin_ptr;
    *begin_ptr = ch;
 
    // update pointers positions
    begin_ptr++;
    end_ptr--;
  }
}
 
// Driver code
int main()
{
 
  // Get the string
  char str[100] = "GeeksForGeeks";
  cout<<"Enter a string: "<<str<<endl;
 
  // Reverse the string
  reverseString(str);
 
  // Print the result
  printf("Reverse of the string: %s\n", str);
 
  return 0;
}
 
// This code is contributed by akashish__

C




#include <stdio.h>
#include <string.h>
 
// Function to reverse the string
// using pointers
void reverseString(char* str)
{
    int l, i;
    char *begin_ptr, *end_ptr, ch;
 
    // Get the length of the string
    l = strlen(str);
 
    // Set the begin_ptr and end_ptr
    // initially to start of string
    begin_ptr = str;
    end_ptr = str;
 
    // Move the end_ptr to the last character
    for (i = 0; i < l - 1; i++)
        end_ptr++;
 
    // Swap the char from start and end
    // index using begin_ptr and end_ptr
    for (i = 0; i < l / 2; i++) {
 
        // swap character
        ch = *end_ptr;
        *end_ptr = *begin_ptr;
        *begin_ptr = ch;
 
        // update pointers positions
        begin_ptr++;
        end_ptr--;
    }
}
 
// Driver code
int main()
{
 
    // Get the string
    char str[100] = "GeeksForGeeks";
    printf("Enter a string: %s\n", str);
 
    // Reverse the string
    reverseString(str);
 
    // Print the result
    printf("Reverse of the string: %s\n", str);
 
    return 0;
}

Output:

Enter a string: GeeksForGeeks
Reverse of the string: skeeGroFskeeG

Time Complexity: O(N), where N represents the length of the given string.
Auxiliary Space: O(1), no extra space is required, so it is a constant.


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