137. Single Number II.cpp

/*
Given an integer array nums where every element appears three times except for one, which appears exactly once. Find the single element and return it.

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:
Input: nums = [2,2,3,2]
Output: 3

Example 2:
Input: nums = [0,1,0,1,0,1,99]
Output: 99

Constraints:
1 <= nums.length <= 3 * 104
-231 <= nums[i] <= 231 - 1
Each element in nums appears exactly three times except for one element which appears once.
*/

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        unordered_map<int, int> countMap;
        int result = 0;
        for (int num : nums) {
            countMap[num]++;
        }
        for (auto pair : countMap) {
            if (pair.second == 1) {
                result = pair.first;
                break;
            }
        }
        return result;
    }
};