438.Find All Anagrams in a String

Tags: Medium Two Pinters Hash Table

Links: https://leetcode.com/problems/find-all-anagrams-in-a-string/

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Given a string s and a non-empty string p, find all the start indices of p's anagrams in s.

Strings consists of lowercase English letters only and the length of both strings s and p will not be larger than 20,100.

The order of output does not matter.

Example 1:

Input:
s: "cbaebabacd" p: "abc"

Output:
[0, 6]

Explanation:
The substring with start index = 0 is "cba", which is an anagram of "abc".
The substring with start index = 6 is "bac", which is an anagram of "abc".

Example 2:

Input:
s: "abab" p: "ab"

Output:
[0, 1, 2]

Explanation:
The substring with start index = 0 is "ab", which is an anagram of "ab".
The substring with start index = 1 is "ba", which is an anagram of "ab".
The substring with start index = 2 is "ab", which is an anagram of "ab".

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虽然官方给的标签是Hash Table，但是一般限制都是小写字母的时候，可以不用哈希表，而是用一个长度为26的数组来代替，这样速度会快很多。

本题题意是考虑给定的模式串p，先用pattern统计出每个字符的频率，因为题目限定不用考虑顺序，如果考虑顺序，那就是KMP了。

双指针维护长度为p的窗口，每次start和end往前移动一个位置，相应的需要减去被去掉的start和增加的end，在增加end的时候，需要判断是否还在字符串的范围内。

比较两个字符频率数组时间复杂度是$O(1)$，总的时间复杂度是$O(1)$，空间复杂度是$O(1)$。