题目描述

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

定义一个map和set，map的key是s中的字符，value的t中相应字符，map是s字符映射到t字符。set存放map的value。

如果s中的某个字符，在map的key中出现了，那么如果value不是t对应字符，返回false。否则查看set中是否存在。

代码

    public static boolean isIsomorphic(String s, String t) {

        if (s == null || t == null) {
            return false;
        }

        if (s.length() != t.length()) {
            return false;
        }

        Map<Character, Character> map = new HashMap<Character, Character>();
        Set<Character> set = new HashSet<Character>();

        for (int i = 0; i < s.length(); i++) {
            char c1 = s.charAt(i);
            char c2 = t.charAt(i);

            if (map.containsKey(c1)) {
                if (map.get(c1) != c2) {
                    return false;
                }
            } else {
                if (set.contains(c2)) {
                    return false;
                }
                map.put(c1, c2);
                set.add(c2);
            }
        }

        return true;
    }