* 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.