LeetCode 438. Find All Anagrams in a String

Description

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

Given two strings s and p, return an array of all the start indices of p‘s anagrams in s. You may return the answer in any order.

An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

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".

Constraints:

  • 1 <= s.length, p.length <= 3 * 104
  • s and p consist of lowercase English letters.

Explanation

Iterate the string s with a pointer i and find out all the positions where s[i: i+len(p)] has the same counter as string p.

Python Solution

class Solution:
    def findAnagrams(self, s: str, p: str) -> List[int]:
        results = []
        
        p_set = collections.Counter(p)
        len_p = len(p)
        
        for i in range(len(s) - len_p + 1):
            sub = s[i : i + len_p]
            
            if collections.Counter(sub) == p_set:
                results.append(i)
                                
        return results
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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