Leetcode: Isomorphic Strings

Given two strings s and t, determine if they are isomorphic.

Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.

For example,
Given "egg", "add", return true.

Given "foo", "bar", return false.

Given "paper", "title", return true.

Note:
You may assume both s and t have the same length.

 1 public class Solution {
 2     public boolean isIsomorphic(String s, String t) {
 3         if (s==null || t==null || s.length() != t.length()) return false;
 4         HashMap<Character, Character> map = new HashMap<Character, Character>();
 5         for (int i=0; i<s.length(); i++) {
 6             if (!map.containsKey(s.charAt(i))) {
 7                 if (!(map.values().contains(t.charAt(i)))) {
 8                     map.put(s.charAt(i), t.charAt(i));
 9                 }
10                 else return false;
11             }
12             else if (map.get(s.charAt(i)) != t.charAt(i)) return false;
13         }
14         return true;
15     }
16 }