LeetCode 904. 水果成篮

class Solution {
public:
    int totalFruit(vector<int>& tree) {
        int res = 0;
        unordered_map<int, int> cnt;
        for (int i = 0, j = 0, s = 0; i < tree.size(); i ++ ) {
            if ( ++ cnt[tree[i]] == 1) s ++ ;
            while (s > 2) {
                if ( -- cnt[tree[j]] == 0) s -- ;
                j ++ ;
            }
            res = max(res, i - j + 1);
        }
        return res;
    }
};