Leetcode - Isomorphic Strings

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.

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Example

Given “egg”, “add”, return true.

Given “foo”, “bar”, return false.

Given “paper”, “title”, return true.

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Note

You may assume both s and t have the same length.

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Java Code

//版本一：使用HashMap存储字符之间的映射关系public boolean isIsomorphic(String s, String t) {   int len = s.length();   if(len < 2) return true;   //把<s, t>作为键值对存入HashMap中   HashMap<Character, Character> map = new HashMap<>(2*len);   //字符串转字符数组   char[] sKey = s.toCharArray();   char[] tValue = t.toCharArray();   for (int i = 0; i < len; i++) {        if(map.containsKey(sKey[i])) {            //如果map中已经存在当前字符sKey[i]且其映射的字符与当前tValue[i]不同，返回假            if(tValue[i] != map.get(sKey[i]))                return false;        }else {            //如果map中不存在当前字符sKey[i]但已经有其他的键映射到当前的值tValue[i]，返回假            if(map.containsValue(tValue[i]))                return false;            //如果map中不存在当前的键值对<sKey[i], tValue[i]>，则存入            map.put(sKey[i], tValue[i]);        }    }    return true;}//版本二：使用数组记录原字符和映射字符的状态并存储字符之间的映射关系public boolean isIsomorphic(String s, String t) {    boolean [] key = new boolean[128];//默认初始化为false，用于标记各个key是否存在    boolean[] value = new boolean[128];//默认初始化为false，用于标记各个value是否存在    int[] map = new int[128];//默认初始化为0    char[] sKey = s.toCharArray();    char[] tValue = t.toCharArray();    char ss, tt;    for(int i = s.length() - 1; i >= 0; --i) {        ss = sKey[i];        tt = tValue[i];        if(key[ss]) {            //如果ss在s中已经出现过且之前映射的字符与当前映射的字符tt不同，返回假            if(map[ss] != tt) return false;        }else {            //如果ss在s中未出现过但已有其他key映射到了tt，返回假            if(value[tt]) return false;            //否则标记key和value，并存储它们之间的映射关系            key[ss] = true;            value[tt] = true;            map[ss] = tt;        }    }    return true;    }