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.

用hashmap，key是s[i]，value是t[i]

注意：如果map中已经存在s[i]这个key，不能再更新其value；由于一个value也只能对应一个key，如果map中已经存在s[i]对应的value，则无法添加新的映射到value的key

时间：O(N)，空间：O(N)

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