题目地址：

https://www.lintcode.com/problem/substring-with-at-least-k-distinct-characters/description

给定一个字符串sss，求其包含至少kkk个不同字符的子串的个数。

思路是双指针。开两个指针iii和jjj，其中iii是右指针，jjj是左指针。我们累加以jjj为左端点的满足条件的子串个数。当s[j:i]s[j:i]s[j:i]这个区间里包含至少kkk个不同字符时，此时∀k≥i,s[j:k]\forall k \ge i, s[j:k]∀k≥i,s[j:k]都成立，所以累加ls−il_s-ils−i，此时左端点为jjj的情况已经枚举完毕，则右移jjj，继续做累加，直到区间不满足条件为止，则右移iii。代码如下：

import java.util.HashMap;
import java.util.Map;public class Solution {/*** @param s: a string* @param k: an integer* @return: the number of substrings there are that contain at least k distinct characters*/public long kDistinctCharacters(String s, int k) {// Write your code hereMap<Character, Integer> map = new HashMap<>();long res = 0;for (int i = 0, j = 0; i < s.length(); i++) {map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);while (map.size() >= k) {res += s.length() - i;char ch = s.charAt(j);map.put(ch, map.get(ch) - 1);if (map.get(ch) == 0) {map.remove(ch);}j++;}}return res;}
}

时空复杂度O(ls)O(l_s)O(ls)。

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【Lintcode】1375. Substring With At Least K Distinct Characters

题目地址：

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