Find the XOR of Numbers Which Appear Twice - Problem

You are given an array nums, where each number in the array appears either once or twice.

Return the bitwise XOR of all the numbers that appear twice in the array, or 0 if no number appears twice.

Input & Output

Example 1 — Basic Case

$ Input: nums = [1,2,3,2]

› Output: 2

💡 Note: Only the number 2 appears twice in the array, so we return 2

Example 2 — Multiple Duplicates

$ Input: nums = [1,2,1,3]

› Output: 1

💡 Note: Only the number 1 appears twice, so we return 1

Example 3 — No Duplicates

$ Input: nums = [1,2,3]

› Output: 0

💡 Note: No number appears twice, so we return 0

Constraints

1 ≤ nums.length ≤ 50
1 ≤ nums[i] ≤ 50

Visualization

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Asked in

G Google 15 M Microsoft 12

The key insight is to identify numbers that appear exactly twice and XOR them together. Best approach uses a hash set to track seen numbers and XOR duplicates in a single pass. Time: O(n), Space: O(n)

Common Approaches

✓ Brute Force - Count Occurrences

⏱️ Time: O(n²) Space: O(1)

Use nested loops to count occurrences of each number. For every number that appears exactly twice, include it in the XOR result.

Frequency Count with Hash Map

⏱️ Time: O(n) Space: O(n)

Use a hash map to count the frequency of each number in a single pass. Then iterate through the frequency map to find all numbers that appear exactly twice and XOR them together.

Optimized Bit Manipulation

⏱️ Time: O(n) Space: O(n)

Leverage the mathematical property that XOR of a number with itself twice equals the number (a ⊕ a ⊕ a = a). Track seen numbers using XOR operations to identify duplicates.

Brute Force - Count Occurrences — Algorithm Steps

Iterate through each number in the array
Count how many times this number appears in the entire array
If count is 2, XOR it with the result

Visualization

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Step-by-Step Walkthrough

Check Each Number

For each position, count how many times that number appears

Count Occurrences

Scan entire array to count occurrences of current number

XOR Duplicates

If count equals 2, XOR the number with result

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_VAL 50

int solution(int* nums, int numsSize) {
    int freq[MAX_VAL + 1] = {0};
    
    // Count frequency of each number
    for (int i = 0; i < numsSize; i++) {
        freq[nums[i]]++;
    }
    
    int result = 0;
    // XOR all numbers that appear exactly twice
    for (int i = 1; i <= MAX_VAL; i++) {
        if (freq[i] == 2) {
            result ^= i;
        }
    }
    
    return result;
}

int main() {
    char line[10000];
    fgets(line, sizeof(line), stdin);
    
    // Parse array format [1,2,3]
    int nums[1000];
    int size = 0;
    
    char* ptr = strchr(line, '[');
    if (ptr) {
        ptr++;
        char* token = strtok(ptr, ",]");
        while (token != NULL) {
            nums[size++] = atoi(token);
            token = strtok(NULL, ",]");
        }
    }
    
    printf("%d\n", solution(nums, size));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n²)

Nested loops - for each element, scan entire array to count occurrences

✓ Linear Growth

Space Complexity

O(1)

Only using variables for counting and result

⚡ Linearithmic Space

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Find the XOR of Numbers Which Appear Twice - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force - Count Occurrences — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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