LeetCode OJ 205 Isomorphic Strings

Given two strings s andt, 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.

题意：判断两个字符串同形字符串，就是说，将s中的字符替换为t中的相应字符，能够保证s中不同的字符替换成t中的不同的字符，可以想象成一个加密过程，利用某种映射，将s加密成t

public class Solution {    public boolean isIsomorphic(String s, String t) {        //利用map保存映射关系        Map<Character,Character> map = new HashMap<Character,Character>();        for(int i = 0 ; i < s.length() ; i ++){            char key = s.charAt(i);            char value = t.charAt(i);            if(map.containsKey(key)){                if(value != map.get(key)){//存在一条key->x的映射，且x != value                    return false;                }            }else if(map.containsValue(value)){                return false;//已经存在一条x->value的映射,但是x != key            }else{//不存在key->x映射和x->value的映射                map.put(key,value);            }        }        return true;    }}

搜了一下，感觉别人的算法跟我的差不多，没找到很高效的算法，为什么我的运行时间那么慢