LeetCode – Permutation in String (Java)

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Given two strings s1 and s2, write a function to return true if s2 contains the permutation of s1. In other words, one of the first string’s permutations is the substring of the second string.

For example:

Input: s1 = "ab" s2 = "eidbaooo"
Output: True
Explanation: s2 contains one permutation of s1 ("ba").

Java Solution

public boolean checkInclusion(String s1, String s2) {
    HashMap<Character, Integer> dict = new HashMap<>();
    for (int i = 0; i < s1.length(); i++) {
        int feq = dict.getOrDefault(s1.charAt(i), 0);
        dict.put(s1.charAt(i), feq + 1);
    }
 
    HashMap<Character, Integer> temp = new HashMap<>();
    int i = 0;
    for (int j = 0; j < s2.length(); j++) {
        if (!dict.containsKey(s2.charAt(j))) {
            i = j + 1;
            temp.clear(); //clear counter
            continue;
        }
 
        int count = temp.getOrDefault(s2.charAt(j), 0);
        if (count == 0 || count < dict.get(s2.charAt(j))) {
            temp.put(s2.charAt(j), count + 1);
 
            if (j - i + 1 == s1.length()) {
                return true;
            }
        } else {
            while (i < j) {
                if (s2.charAt(i) == s2.charAt(j)) {
                    i++;
                    break;
                }
 
                temp.put(s2.charAt(i), temp.get(s2.charAt(i)) - 1);
                i++;
            }
        }
    }
 
    return false;
}

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