Longest Subarray Length

• Time : $O(n)$
• Space : $O(n)$

int Solution::solve(vector<int> &A) {
    int res = 0, now = 0;
    unordered_map<int, int> freq{{0,-1}};
    for(int i = 0; i < A.size(); i++) {
        now += A[i] ? 1 : -1;
        if(!freq.count(now)) freq[now] = i;
        if(freq.count(now - 1)) res = max(res, i - freq[now-1]);
    }
    return res;
}