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.

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        if(s == null || t == null){
            return false;
        }
        else if(s.length() != t.length()){
            return false;
        }
        HashMap<Character, Character> table=new HashMap<Character, Character>();
        int len=s.length();
        for(int i=0; i<len; i++){
            Character sc=s.charAt(i);
            Character tc=t.charAt(i);
            if(table.containsKey(sc)){
                if(table.get(sc) != tc){
                    return false;
                }
            }
            else{
                //table.put(sc, tc);
                if(table.containsValue(tc)){
                    return false;
                }
                else{
                    table.put(sc,tc);
                }
            }
        }
        return true;
    }
}