At first beginning, I was trying to use hash set to record the characters. But I found that was wrong.

Because if there are couple same chars, when you erase it, you lost all the information and cause the result wrong.

 

Note:

size > 2 not size > 1

class Solution {public:    int lengthOfLongestSubstringTwoDistinct(string s) {        int len = s.size(), result = 0;        if (len < 2) return len;        unordered_map
     
       mapping;        for (int left = 0, right = 0; right < len; right++) {            if (mapping.find(s[right]) != mapping.end()) {                mapping[s[right]]++;            } else {                mapping[s[right]] = 1;                while (mapping.size() > 2) {                    char tmp = s[left++];                    if (mapping[tmp] > 1) mapping[tmp]--;                    else mapping.erase(tmp);                }            }            result = max(result, right - left + 1);        }        return result;    }};