README_EN.md

16.01. Swap Numbers

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Description

Write a function to swap a number in place (that is, without temporary vari­ ables).

Example:

Input: numbers = [1,2]

Output: [2,1]

Note:

• numbers.length == 2

Solutions

Solution 1: Bitwise Operation

We can use the XOR operation $\oplus$ to implement the swap of two numbers.

The XOR operation has the following three properties:

• Any number XORed with $0$ remains unchanged, i.e., $a\oplus 0=a$.
• Any number XORed with itself results in $0$, i.e., $a\oplus a=0$.
• The XOR operation satisfies the commutative and associative laws, i.e., $a\oplus b\oplus a=b\oplus a\oplus a=b\oplus (a\oplus a)=b\oplus 0=b$.

Therefore, we can perform the following operations on two numbers $a$ and $b$ in the array $numbers$:

• $a=a\oplus b$, now $a$ stores the XOR result of the two numbers;
• $b=a\oplus b$, now $b$ stores the original value of $a$;
• $a=a\oplus b$, now $a$ stores the original value of $b$;

In this way, we can swap two numbers without using a temporary variable.

The time complexity is $O(1)$, and the space complexity is $O(1)$.

Python3

class Solution:
    def swapNumbers(self, numbers: List[int]) -> List[int]:
        numbers[0] ^= numbers[1]
        numbers[1] ^= numbers[0]
        numbers[0] ^= numbers[1]
        return numbers

Java

class Solution {
    public int[] swapNumbers(int[] numbers) {
        numbers[0] ^= numbers[1];
        numbers[1] ^= numbers[0];
        numbers[0] ^= numbers[1];
        return numbers;
    }
}

TypeScript

function swapNumbers(numbers: number[]): number[] {
    numbers[0] ^= numbers[1];
    numbers[1] ^= numbers[0];
    numbers[0] ^= numbers[1];
    return numbers;
}

C++

class Solution {
public:
    vector<int> swapNumbers(vector<int>& numbers) {
        numbers[0] ^= numbers[1];
        numbers[1] ^= numbers[0];
        numbers[0] ^= numbers[1];
        return numbers;
    }
};

Go

func swapNumbers(numbers []int) []int {
	numbers[0] ^= numbers[1]
	numbers[1] ^= numbers[0]
	numbers[0] ^= numbers[1]
	return numbers
}

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