LeetCode - Permutation in String

Question Definition

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.

Example 1:

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

Example 2:

Input:s1= "ab" s2 = "eidboaoo"
Output: False

Note:

1. The input strings only contain lower case letters.
2. The length of both given strings is in range [1, 10,000].

   Java Solution

   public boolean checkInclusion(String s1, String s2) {
    int n1 = s1.length(), n2 = s2.length(), cnt = n1, left = 0;
    int[] m = new int[128];
    for (char c : s1.toCharArray())
        m[c]++;
    for (int right = 0; right < n2; right++) {
        if (m[s2.charAt(right)]-- > 0)
            --cnt;
        while (cnt == 0) {
            if (right - left + 1 == n1) return true;
            if (++m[s2.charAt(left++)] > 0) ++cnt;
        }
    }
    return false;
   }