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438. Find All Anagrams in a String

程序员文章站 2022-07-12 08:38:14
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原题

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".

代码实现

这个算法的时间复杂度为O(n),空间复杂度为O(1)。这个算法比较难理解,以后慢慢回头琢磨吧。

  public IList<int> FindAnagrams(string s, string p){
            IList<int> rtn = new List<int>();
            int[] hash = new int[123]; //a~z: 97~122
            foreach (var c in p){
                hash[Convert.ToInt32(c)]++; //hash: key:char, value: occuring times
            }
            int eachBeg = 0, eachEnd = 0, count = p.Length;
            while (eachEnd < s.Length){
                char tmpchar = s[eachEnd];
                if (hash[tmpchar] >= 1) 
                    count--;
                hash[tmpchar]--;
                eachEnd++; //every time the eachEnd pointer to move toward right
                if (count == 0) 
                    rtn.Add(eachBeg);
                //reset the hash. 
                if (eachEnd - eachBeg == p.Length){ 
                    char tmp = s[eachBeg];
                    if (hash[tmp] >= 0)
                        count++;
                    hash[tmp]++;
                    eachBeg++;
                }
            }
            return rtn;
        }