Valid Anagram

s = "anagram", t = "nagaram", return true. s = "rat", t = "car", return false.

• Time: O(n)
• Space: O(1)

public boolean isAnagram(String s, String t) {
    if (s == null || t == null || s.length() != t.length()) {
        return false;
    }
    int[] num = new int[26];
    for (int i = 0; i < s.length(); i++) {
        num[s.charAt(i) - 'a']++;
        num[t.charAt(i) - 'a']++;
    }
    for (int i = 0; i < num.length; i++) {
        if (num[i] != 0)
            return false;
    }
    return true;
}