OddOccurrencesInArray

#include <bits/stdc++.h>

using namespace std;

int solution(vector<int> &A)
{
    unordered_map<int, int> countMap;
    for (auto a : A) ++countMap[a];

    for (auto [k, v] : countMap)
    {
        if (v % 2 == 0) continue;
        return k;
    }
}

• T: $O(n)$
• S: $O(n)$