Subarray with given XOR

• Time : $O(n)$
• Space : $O(n)$

int Solution::solve(vector<int> &A, int B) {
    unordered_map<int, int> freq{{0,1}};
    int res = 0, now = 0;
    for(int i = 0; i < A.size(); i++) {
        now ^= A[i];
        res += freq[now ^ B];
        freq[now] += 1;
    }
    return res;
}