[Locked] Longest Substring with At Most Two Distinct Characters

Longest Substring with At Most Two Distinct Characters

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

For example, Given s = “eceba”,

T is "ece" which its length is 3.

分析：

　　最多包含两个不同字母的子串，至少要遍历一遍，复杂度为O(n)，而要实现O(n)复杂度，用移动窗口思想即可。

代码：

int lentwoc(string s) {
    if(s.empty())
        return 0;
    char a = s[0], b = '\0';
    //alast为a字母最后出现的位置，blast为b字母最后出现的位置，lastpos为新两元序列的开端位置，maxl为全局最长序列长度
    int alast = 0, blast = -1, lastpos = 0, maxl = 0;
    for(int i = 1; i < s.length(); i++) {
        //开启新两元序列
        if(s[i] != a && s[i] != b) {
            //记录前一个两元序列长度
            maxl = max(i - lastpos, maxl);
            if(alast < blast) {
                alast = i;
                a = s[i];
                lastpos = blast;
            }
            else {
                blast = i;
                b = s[i];
                lastpos = alast;
            }
        }
        else
            s[i] == a ? alast = i : blast = i;
    }
    return maxl;
}