《leetCode》：Isomorphic Strings

题目

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.

思路

借用HashMap即可完成，但是要注意的是：Map虽然保证了键的唯一，但是我们还要保证键值中的值唯一，即不可以有多个键对应 一个值。

public boolean isIsomorphic(String s, String t) {
        if(s==null&&t==null){
            return true;
        }
        int len1=s.length();
        int len2=t.length();
        if(len1!=len2){
            return false;
        }
        Map<Character,Character> map=new HashMap<Character,Character>();
        StringBuffer sb=new StringBuffer();
        for(int i=0;i<len1;i++){
            char ch=map.getOrDefault(s.charAt(i), '\0');
            if(ch=='\0'){//第一次出现，则保存
                //保存之前要检查，键值中的值是否唯一，即不能多个不同的键映射到一个值
                char tempCh=t.charAt(i);
                if(map.containsValue(tempCh)){
                    return false;
                }
                map.put(s.charAt(i),tempCh);//只有键和值都没有出现，才加入map
                sb.append(tempCh);
            }
            else{
                sb.append(ch);
            }

        }
        if(t.equals(sb.toString())){
            return true;
        }
        return false;
    }