Single Number Problem & Solution

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. You must implement a solution with a linear runtime complexity and use only constant extra space.

See the single number problem on LeetCode.

C++ Solution

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

class Solution {
  int singleNumber(vector<int>& nums) {
    int result = 0;

    for (int i = 0; i < nums.size(); ++i) {

      // Performs a XOR to cancel out numbers that repeat again.
      result ^= nums[i];

    return result;

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