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<char, int> 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; } };
LeetCode – Refresh – Longest Substring with At Most Two Distinct Characters
原文：http://www.cnblogs.com/shuashuashua/p/4352700.html