How to Swap Two Strings Without Using Third String in C++?

Last Updated : 29 Feb, 2024

In C++, a string is a sequence of characters. Swapping two strings typically involves using a third string as temporary storage. However, it’s possible to swap two strings without using a third string. In this article, we will learn how to do this in C++.

Example

Input:
str1 = "Hello"
str2 = "world"

Output:
str1 = "world"
str2 = "Hello"

Swapping Strings Without Using a Third String

The XOR algorithm can be used to swap two strings without using a third string. The XOR operation has the property that

A ^ B ^ B = A

which can be used to swap two variables.

Approach

Iterate through each character of the strings.
Perform bitwise XOR (^) operation on corresponding characters of the strings.
Repeat the operation for all characters, effectively swapping their values.

C++ Program to Swap Strings Without Using Third String

C++

// C++ Program to illustrate how to swap strings without
// using a third-string
#include <iostream>
#include <string>
using namespace std;

// function to SWap two strings Without Using Third Variable
void swapStrings(string& str1, string& str2)
{
    int length1 = str1.length();
    int length2 = str2.length();

    for (int i = 0; i < length1 && i < length2; ++i) {
        str1[i] ^= str2[i];
        str2[i] ^= str1[i];
        str1[i] ^= str2[i];
    }

    if (length1 != length2) {
        for (int i = length1; i < length2; ++i) {
            str1 += str2[i];
        }
        for (int i = length2; i < length1; ++i) {
            str2 += str1[i];
        }
        str1 = str1.substr(0, length2);
        str2 = str2.substr(0, length1);
    }
}

// Driver Code
int main()
{
    string str1 = "Hello";
    string str2 = "World";

    cout << "Before swapping:" << endl;
    cout << "String 1: " << str1 << endl;
    cout << "String 2: " << str2 << endl;

    swapStrings(str1, str2);

    cout << "\nAfter swapping:" << endl;
    cout << "String 1: " << str1 << endl;
    cout << "String 2: " << str2 << endl;

    return 0;
}

Output

Before swapping:
String 1: Hello
String 2: World

After swapping:
String 1: World
String 2: Hello

Time Complexity: O(max(N,M)), where N and M are the sizes of the strings.
Space Complexity: O(1)