438. Find All Anagrams in a String

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".

方法0: Brute force TLE

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        if (s.empty() || p.empty()) return {};
        unordered_map<char, int> hash;
        for (char c: p) hash[c]++;
        unordered_map<char, int> copy(hash);
        vector<int> result;
       
        for (int i = 0; i <= (int)(s.size() - p.size()); i++) {
            copy.clear();
            int j = 0;
            for (; j < p.size(); j ++) {
                copy[s[i + j]]++;
                if (copy[s[i + j]] > hash[s[i + j]]) break;
            }
            if (j == p.size()) result.push_back(i);
        }
        return result;
    }
};

方法1: sliding window

Complexity

Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        if (s.empty() || p.empty()) return {};
        int n = s.size();
        int m = p.size();
        vector<int> hash(26, 0);
        vector<int> copy(26, 0);
        vector<int> result;
        for (char c: p) hash[c - 'a']++;
        for (int i = 0; i < n ; i++) {
            if (i >= p.size()) copy[s[i - m] - 'a']--;
            copy[s[i] - 'a']++;
            if (hash == copy) result.push_back(i - m + 1);
        }
        return result;
    }
};