LeetCode //C - 260. Single Number III

260. Single Number III

Given an integer array nums, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once. You can return the answer in any order.

You must write an algorithm that runs in linear runtime complexity and uses only constant extra space.
 

Example 1:

Example 2:

Example 3:

Constraints:

• 2 < = n u m s . l e n g t h < = 3 ∗ 1 0 4 2 <= nums.length <= 3 * 10^4 2<=nums.length<=3∗104
• − 2 31 < = n u m s [ i ] < = 2 31 − 1 -2^{31} <= nums[i] <= 2^{31} - 1 −231<=nums[i]<=231−1
• Each integer in nums will appear twice, only two integers will appear once.

From: LeetCode
Link: 260. Single Number III

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Solution:

Ideas:

Use of Unsigned Type:

• By casting xor_result to an unsigned int, we avoid the undefined behavior associated with negating the most negative integer.
• The expression -(unsigned int)xor_result safely computes the two’s complement for the unsigned value.
• set_bit now correctly isolates the rightmost set bit without causing any runtime errors.

Code:

/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* singleNumber(int* nums, int numsSize, int* returnSize) {
    int xor_result = 0;
    for (int i = 0; i < numsSize; i++) {
        xor_result ^= nums[i];
    }

    // Cast to unsigned int for safe bit manipulation
    unsigned int set_bit = (unsigned int)xor_result & (-(unsigned int)xor_result);

    int num1 = 0, num2 = 0;
    for (int i = 0; i < numsSize; i++) {
        if ((nums[i] & set_bit) != 0) {
            num1 ^= nums[i];
        } else {
            num2 ^= nums[i];
        }
    }

    int* result = (int*)malloc(2 * sizeof(int));
    result[0] = num1;
    result[1] = num2;
    *returnSize = 2;

    return result;
}