159

class Solution {
public:
    int lengthOfLongestSubstringTwoDistinct(string s) {
        unordered_map<char, int> cnt;
        int n=s.length(), left=0, right=0, ans=0;
        for(right=0; right<n; right++){
            cnt[s[right]]++;
            while(left<right && cnt.size()>2){
                if(--cnt[s[left]] == 0) cnt.erase(s[left]);
                left++;
            }
            ans = max(ans, right-left+1);
        }
        return ans;
    }
};