IsomorphicStrings.java

public boolean isIsomorphic(String a, String b) {
		if (a == null || b == null || a.length() != b.length()) {
			return false;
		}
		Map<Character, Character> lookup = new HashMap<>();
		Set<Character> dupSet = new HashSet<>();

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

			if (lookup.containsKey(c1)) {
				if (c2 != lookup.get(c1)) {
					return false;
				}
			} else {
				lookup.put(c1, c2);
				// this to prevent different c1s map to the same c2, it has to be a bijection mapping
				if (dupSet.contains(c2)) {
					return false;
				}
				dupSet.add(c2);
			}
		}
		return true;
	}