205. 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.

Example 1:

Input: s = "egg", t = "add"
Output: true

Example 2:

Input: s = "foo", t = "bar"
Output: false

Example 3:

Input: s = "paper", t = "title"
Output: true

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

class Solution {
    public boolean isIsomorphic(String s, String t) {
        HashMap<Character, Character> map1 = new HashMap<>();
        HashMap<Character, Character> map2 = new HashMap<>();
        for(int i = 0; i < s.length(); i++){
           char c1 = s.charAt(i);
           char c2 = t.charAt(i);
            if(map1.containsKey(c1)){
                if(map1.get(c1) != c2){
                    return false;
                }
            }
            else {
                map1.put(c1, c2);
            }
            if(map2.containsKey(c2)){
                if(map2.get(c2) != c1){
                    return false;
                }
            }
            else {
                map2.put(c2, c1);
            }
        }
        return true;
    }
}

判断两个string是否同构

abb 和 egg同构

ab和aa 不同构
用两个hashmap来存放对应的char。不同构本质上是因为有一个字母对应不上，意思就是对应不上之前已经有的结构，比如aa和ab，aa已经对应了，那剩下有a出现只能也对应a，所以ab就不同构了。