Java solution with two hash maps and guaranted O(N) running time

  • 0

    Here is the java solution that uses two hash map:

    public class Solution {
    public boolean isIsomorphic(String s, String t) {
        if(s == null || t == null) {
            return false;
        } else if(s.length() != t.length()) {
            return false;
        final Map<Character, Character> chars = new HashMap<Character, Character>();
        final Map<Character, Character> reverse = new HashMap<Character, Character>();
        for(int ind = 0; ind < s.length(); ind++) {
            final char src = s.charAt(ind);
            final char des = t.charAt(ind);
            if(!chars.containsKey(src) && !reverse.containsKey(des)) {
                chars.put(src, des);
                reverse.put(des, src);
            } else {
                if((chars.containsKey(src) && chars.get(src) != des)
                        || (reverse.containsKey(des) && reverse.get(des) != src)) {
                    return false;
        return true;
    • first one is used to store the two coresponding characters
    • second one stores the reversed mapping two guarantee that no two characters maps to the same one i.e. 'aa' is not isomorphic with 'ab'.

  • 1

    Your solution seems legit, but I guess to further optimize the solution you can use two char arrays of size 256 each of the maps.

    Happy coding!

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