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.

分析

1. simplify new HashMap<Character, Character> to new char[256] here. why 256? There are 256 ASCII characters. This includes standard ASCII characters(0-127) and Extended ASCII characters(128-255)
2. traverse the whole string s (or t, because 's and t have the same length'), using two pointers: a, b to check (1)whether character a has marked, if no, mark it to b ; if yes, check whether it equals to b (2) whether character b has marked, if no, mark it to a ; if yes, check whether it equals to a
3. why we should check twice ? Take s: bar t:foo for example, it works from s to t; but doesn't work from t to s

解题

class Solution {
    public boolean isIsomorphic(String s, String t) {
        if(s.length()!=t.length()) return false;

         char [] mapAB = new char[256];
         char [] mapBA = new char[256];
         for(int i = 0; i<s.length();i++){
            char a = s.charAt(i);
            char b = t.charAt(i);

            if(mapAB[a]!=0){
                if(mapAB[a]!=b) return false;
            }else{
                mapAB[a]= b;
            }

            if(mapBA[b]!=0){
                if(mapBA[b]!=a) return false;
            }else{
                mapBA[b]=a;
            }           
        } 
        return true;   
    }
}