LeetCode OJ Single Number II

Given an array of integers, every element appears three times except for one. Find that single one.

Note:

Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

To solve this problem, I used quick_sort to sort the array, and then find...

class Solution {
public:
    int interchange(int A[], int low, int high) {
        int key = A[low];
        while (low < high) {
            while (low < high && A[high] >= key) high--;
            if (low < high) A[low++] = A[high];
            while (low < high && A[low] <= key) low++;
            if (low < high) A[high--] = A[low];
        }
        A[low] = key;
        return low;
    }
    void quicksort(int A[], int low, int high) {
        int new_mid;
        if (low < high) {
            new_mid = interchange(A, low, high);
            quicksort(A, low, new_mid - 1);
            quicksort(A, new_mid + 1, high);
        }
    }
    int singleNumber(int A[], int n) {
        if (n < 3) return A[0];
        
        quicksort(A, 0, n - 1);
        int counter = 1;
        for (int i = 1; i < n; i++) {
            if (A[i] == A[i - 1]) {
                counter++;
            } else {
                if (counter != 3) {
                    return A[i - 1];
                } else {
                    counter = 1;
                }
            }
        }
        return A[n - 1];
    }
};