340. Longest Substring with At Most K Distinct Characters

Description

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

For example, Given s = “eceba” and k = 2,

T is "ece" which its length is 3.

Solution

Slide window, O(n), S(1)

很直观的做法，tricky的地方是，如果当前distinct > k，需要将winStart向后移动，直到某个char完全被清空就行了。注意这里是某个，而非原始winStart指向的char！看这个testcase：

"abacc", k = 2

结果返回的是3，而非2.=。

class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s == null || s.isEmpty() || k < 1) {
            return 0;
        }
        
        int[] count = new int[256];
        int distinct = 0;
        int start = 0;
        int maxLen = 0;
        
        for (int end = 0; end < s.length(); ++end) {
            char c = s.charAt(end);
            if (count[c]++ == 0) {
                ++distinct;
            }
            
            if (distinct > k) {
                while (--count[s.charAt(start++)] > 0) { }
                --distinct;
            }
            
            maxLen = Math.max(end - start + 1, maxLen);
        }
        
        return maxLen;
    }
}