LeetCode 137. Single Number II

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class Solution {
public:
    int singleNumber(vector<int>& nums) {
        unordered_map<int, int> freq;
        for (const auto& num: nums)
            ++freq[num];
        int ans = 0;
        for (auto [num, occ]: freq)
            if (occ == 1) {
                ans = num;
                break;
            }
        return ans;
    }
};
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class Solution {
public:
    int singleNumber(vector<int>& nums) {
        int ans = 0;
        for (int i = 0; i < 32; ++i) {
            int sum = 0;
            for (const auto& num : nums) {
                sum += (num >> i) & 1;
            }
            if (sum % 3)
                ans |= (1 << i);
        }
        return ans;
    }
};