205. Isomorphic Strings (Easy)

https://leetcode.com/problems/isomorphic-strings/

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.

Example 1:

Input: s = "egg", t = "add"
Output: true

Example 2:

Input: s = "foo", t = "bar"
Output: false

Example 3:

Input: s = "paper", t = "title"
Output: true

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

Solutions

class Solution {

    public boolean isIsomorphic(String s, String t) {
        if (s == null || t == null) {
            return false;
        }

        int len = s.length();

        // Used to keep the distribution of different chars.
        Map<Character, List<Integer>> map1 = new HashMap<>();
        Map<Character, List<Integer>> map2 = new HashMap<>();

        for (int i = 0; i < len; i++) {
            if (!map1.containsKey(s.charAt(i))) {
                map1.put(s.charAt(i), new ArrayList<>());
            }

            map1.get(s.charAt(i)).add(i);
        }

        for (int i = 0; i < len; i++) {
            if (!map2.containsKey(t.charAt(i))) {
                map2.put(t.charAt(i), new ArrayList<>());
            }

            map2.get(t.charAt(i)).add(i);
        }

        Set<Character> visited = new HashSet<>();

        for (int i = 0; i < len; i++) {

            // Important, ignore visited char's distribution to save up time
            if (visited.contains(s.charAt(i))) {
                continue;
            }

            String idx1 = map1.get(s.charAt(i)).toString();
            String idx2 = map2.get(t.charAt(i)).toString();
            if (!idx1.equals(idx2)) {
                return false;
            }

            visited.add(s.charAt(i));
        }

        return true;
    }
}

Incorrect Solutions

class Solution {

    // Time Limit Exceeded

    public boolean isIsomorphic(String s, String t) {
        if (s == null || t == null) {
            return false;
        }

        int len = s.length();

        for (int i = 0; i < len; i++) {
            for (int j = i + 1; j < len; j++) {
                if (s.charAt(i) == s.charAt(j) && t.charAt(i) != t.charAt(j)) {
                    return false;
                }

                if (s.charAt(i) != s.charAt(j) && t.charAt(i) == t.charAt(j)) {
                    return false;
                }
            }
        }

        return true;
    }
}

References