滑动窗口：

class Solution {
public:int totalFruit(vector<int>& fruits) {int max_len = 0;int left = 0;unordered_map<int,int>mp;for(int right = 0; right < fruits.size();right++){mp[fruits[right]]++;while(mp.size() > 2){mp[fruits[left]]--;if(mp[fruits[left]] == 0){mp.erase(fruits[left]);}left++;}max_len = max(max_len, right - left + 1);}return max_len;}
};