205. Isomorphic Strings

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Problem

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

Code

class Solution {
    public boolean isIsomorphic(String s, String t) {
        if(s == null || t == null) return false;
        HashMap<Character, Character> map = new HashMap<>();

        for(int i = 0; i < s.length(); i++){
            char a = s.charAt(i);
            char b = t.charAt(i);
            if(map.containsKey(a)){
                if(map.get(a) == b){
                    continue;
                } else {
                    return false;
                }
            } else {
                // 注意检查看是都这个value已经存在了
                if(!map.containsValue(b)){
                    map.put(a, b);
                } else {
                    return false;
                }
            }
        }
        return true;
    }
}