Isomorphic Strings - Java implementation

IsomorphicStrings.java

package megha.codingproblems.strings;

import java.util.HashMap;
import java.util.Map;

/**
 * Git source URL - https://github.com/meghakrishnamurthy/CodingProblems/blob/master/src/megha/codingproblems/strings/IsomorphicStrings.java
 * Program to find if 2 given alphabetic strings are isomorphic.
 * Two strings are isomorphic if the characters in string A can be replaced to get string B
 * Time complexity - O(n)
 * Space complexity - O(1)
 * 
 * @author megha krishnamurthy
 *
 */
public class IsomorphicStrings {

	/**
	 * Given 2 strings as input determine if they are isomorphic or not
	 * @param a
	 * @param b
	 * @return
	 */
	public boolean isIsomorphic(String a, String b) {
		if(a == null || b == null) {
			return false;
		}
		if(a.length() != b.length()) {
			return false;
		}
		Map<Character, Character> characterMap = new HashMap<Character, Character>();
		for(int i=0; i<a.length(); i++) {
			char c1 = a.charAt(i);
			char c2 = b.charAt(i);
			if(characterMap.containsKey(c1)) {
				if(characterMap.get(c1) != c2) {
					return false;
				}
			} else {
				if(characterMap.containsValue(c2)) {
					return false;
				}
				characterMap.put(c1, c2);
			}
		}
		return true;
	}
	
	public static void main(String args[]) {
		IsomorphicStrings iso = new IsomorphicStrings();
		String a = "good";
	    String b = "deer";
	    
	    String c = "seeds";
	    String d = "flags";
	    
	    String e = "good";
	    String f = "dder";
	    
	    String g = "hello";
	    String h = "hi";
	    
	    System.out.println(iso.isIsomorphic(a,b));
	    System.out.println(iso.isIsomorphic(c,d));
	    System.out.println(iso.isIsomorphic(e,f));
	    System.out.println(iso.isIsomorphic(g,h));
	}
}