LeetCode #159 - Longest Substring with At Most Two Distinct Characters

题目描述：

Given a string s , find the length of the longest substring t  that contains at most 2 distinct characters.

Example 1:

Input: "eceba"

Output: 3

Explanation: t is "ece" which its length is 3.

Example 2:

Input: "ccaabbb"

Output: 5

Explanation: t is "aabbb" which its length is 5.

滑动窗口解决，移动终点，保证窗口中最多只有两种字符，如果超过两种字符，移动起点。

class Solution {
public:
    int lengthOfLongestSubstringTwoDistinct(string s) {
        int n=s.size();
        int i=0;
        int j=0;
        unordered_map<char,int> hash;
        int max_length=0;
        while(j<n)
        {
            if(hash.count(s[j])>0) hash[s[j]]++;
            else
            {
                hash[s[j]]=1;
                if(hash.size()>2) 
                {
                    while(hash.size()>2&&i<=j)
                    {
                        hash[s[i]]--;
                        if(hash[s[i]]==0) hash.erase(s[i]);
                        i++;
                    }
                }
            }
            max_length=max(max_length,j-i+1);
            j++;
        }
        return max_length;
    }
};