LeetCode 159. Longest Substring with At Most Two Distinct Characters

原题链接在这里：https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/

题目：

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.

题解：

与Longest Substring Without Repeating Characters类似。快慢指针维护substring的方法在Minimum Window Substring里有总结.

runner 扫过 char的frequency加一. 条件count++.

当count > 2后移动walker直到count减到2

Time Complexity: O(n), 窗口只移动一遍. Space: O(1). map size.

AC Java:

public class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        int res = 0;
        int [] map = new int[256];
        int walker = 0;
        int runner = 0;
        int count = 0;
        while(runner < s.length()){
            if(map[s.charAt(runner++)]++ == 0){
                count++;
            }
            while(count > 2){
                if(map[s.charAt(walker++)]-- == 1){
                    count--;
                }
            }
            res = Math.max(res, runner - walker);
        }
        return res;
    }
}

类似Longest Substring with At Most K Distinct Characters, Fruit Into Baskets, Max Consecutive Ones II.