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Find all anagrams in a string

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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.

public List<Integer> findAnagrams(String s, String p) {
    List<Integer> list = new ArrayList<>();
    if (s == null || s.length() == 0 || p == null || p.length() == 0) return list;
    int[] hash = new int[256]; //character hash
    //record each character in p to hash
    for (char c : p.toCharArray()) {
        hash[c]++;
    }
    //two points, initialize count to p's length
    int left = 0, right = 0, count = p.length();
    while (right < s.length()) {
        //move right every time, if the character exists in p's hash, decrease the count
        //current hash value >= 1 means the character is existing in p
        if (hash[s.charAt(right++)]-- >= 1) count--; 
        
        //when the count is down to 0, means we found the right anagram
        //then add window's left to result list
        if (count == 0) list.add(left);
    
        //if we find the window's size equals to p, then we have to move left (narrow the window) to find the new match window
        //++ to reset the hash because we kicked out the left
        //only increase the count if the character is in p
        //the count >= 0 indicate it was original in the hash, cuz it won't go below 0
        if (right - left == p.length() && hash[s.charAt(left++)]++ >= 0) count++;
    }
    return list;
}

Posted by Jamie Meyer 3 months ago

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