Determine the number with odd frequency using sort in C++

#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    int res = -1;
    int i = 0;
    int j = 0;
    // create two pointersL i and j
    // i = start pointer of the element
    // j = moving pointer of the element
    // once j reaches the last occurrence of the element,
    // get the difference between j and i (frequency)
    for (i = 0; i < (int) a.size(); i = j) {
        while (j < (int) a.size() && a[i] == a[j]) {
            j++;
        }
        if ((j - i) & 1) {
            res = a[i];
        }
    }
    cout << res << '\n';
    return 0;
}