[Amazon] Longest Substring with At Most K Distinct Characters

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

Example

For example, Given s = "eceba", k = 3,

T is "eceb" which its length is 4.

Challenge 

O(n), n is the size of the string s.

思路：right指针遍历先统计到第一个使map.size()>k的字符，这时候，left指针从头开始遍历，直到可以使map.size()=k

           最长的位置就是left到right，因此最长的个数就是right-left+1

public class Solution {    /**     * @param s : A string     * @return : The length of the longest substring      *           that contains at most k distinct characters.     */    public int lengthOfLongestSubstringKDistinct(String s, int k) {        Map<Character,Integer> map=new HashMap<>();        int longest=0;        int left=0;        for(int right=0;right<s.length();right++){            char ch=s.charAt(right);            if(!map.containsKey(ch)){                map.put(ch,1);            }else{                map.put(ch,map.get(ch)+1);            }            while(map.size()>k){                char leftChar=s.charAt(left);                map.put(leftChar,map.get(leftChar)-1);        //遍历left，直到可以使map.size()减少1                if(map.get(leftChar)==0){                     //也就是说right被统计过的字符                    map.remove(leftChar);                     //哪个的个数先减小到0，就停止                }                left++;            }            longest=Math.max(longest,right-left+1);        }                return  longest;    }}