查找数组中的奇数次出现

I am trying to solve the following codility problem in JS, and I want to achieve a decent Big-O notation score. https://app.codility.com/programmers/lessons/2-arrays/odd_occurrences_in_array/

I'm completely stuck. I cannot figure out a way to do it without writing two loops. This gets me O(N**2) for the complexity, which is confusing as at most it is O(N + N), right?

function solution(A) {
    if(A.length === 1 || !A){
        return A[0];
    }
    const counts = {};
    for(let i in A){
        if(!counts[A[i]]){
            counts[A[i]] = 1
        } else {
            counts[A[i]] = 2
        }
    }
    return +getNotDivisibleByTwo(counts);
}

function getNotDivisibleByTwo(counts) {
    return Object.keys(counts).find(key => counts[key] % 2 !== 0);
}

这个解决方案给了我55％的支持，并不是很好。有解决这个问题的更好方法吗？