[LeetCode]Isomorphic Strings

转载

byamao1 2023-02-02 21:26:18

Question

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.

本题难度Easy。

哈希法

复杂度

时间 O(N) 空间 O(N)

思路

用一个哈希表记录字符串s中字母到字符串t中字母的映射关系，一个集合记录已经映射过的字母（或者用两个哈希表记录双向的映射关系）这里不能只用一个哈希表，因为要排除egg->ddd这种多对一的映射。

代码

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        //require
        Set<Character> set=new HashSet<>();
        Map<Character,Character> map=new HashMap<>();
        char[] schars=s.toCharArray(),tchars=t.toCharArray();
        //invariant
        for(int i=0;i<s.length();i++){
            char sc=schars[i],tc=tchars[i];
            if(map.containsKey(sc)){
                // 如果已经给s中的字符建立了映射，检查该映射是否和当前t中字符相同
                if(map.get(sc)!=tc)return false;
            }else{
                // 如果已经给t中的字符建立了映射，就返回假，因为出现了多对一映射
                if(set.contains(tc))return false;
                else{
                    map.put(sc,tc);
                    set.add(tc);
                }
            }
        }
        //ensure
        return true;
    }
}