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.

public class Solution {
public boolean isIsomorphic(String s, String t) {
if(s == null || t == null)
return false;
if(s.length() == 0 && t.length() == 0)
return true;
if(s.length() != t.length())
return false;

Map<Character, Character> map = new HashMap<>();
Set<Character> set = new HashSet<>();
for(int i = 0; i < s.length(); i++){
Character sChar = s.charAt(i);
Character tChar = t.charAt(i);
if(!map.containsKey(sChar)){
if(!set.contains(tChar)){
set.add(tChar);
map.put(sChar, tChar);
}
else
return false;
}
else{
Character mappedValue = map.get(sChar);
if(!mappedValue.equals(tChar))
return false;
}
}
return true;
}
}