anagram.java

public class IsAnagram {

  boolean anagram(String s1, String s2) {
    if(s1.length() != s2.length()) {
      return false;
    }
    int[] counts = new int[26];
    for(int i=0; i<s1.length(); i++) {
      //'a' - 'a' -> 0, 'b' - 'a' -> 1 ....
      counts[s1.charAt(i)-'a']++;
    }

    for(int i=0; i<s1.length(); i++) {
      counts[s2.charAt(i)-'a']--;
      if(counts[s2.charAt(i)-'a'] < 0)) {
        return false;
      }
    }

    return true;
  }
}

m = s1#length
n = s2#length
Time complexity O(m+n)
space complexity O(1), fixed size int[]