cc150.1.3.java

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

/**
 * 
 * @author james
 * 1.3 Given two strings, write a method to decide if one is a permutation of the other.
 * O(N) time, O(1) space
 */
public class PermutationOfString {

	public static boolean solution(String s1, String s2) {
		if(s1 == null || s2 == null) {
			return false;
		}
		
		if (s1.length() != s2.length()) {
			return false;
		}
		
		Map<Character, Integer> counter = new HashMap<>();
		
		//using first string to populate counter table 
		for (int i = 0; i < s1.length(); ++i) {
			char c = s1.charAt(i);
			if (counter.containsKey(c)) {
				counter.put(c, counter.get(c) + 1);
			} else {
				counter.put(c, 1);
			}
		}
		
		//decrease counter for every character
		for (int i = 0; i < s2.length(); ++i) {
			char c = s2.charAt(i);
			if (!counter.containsKey(c) || counter.get(c) <= 0) {
				return false;
			}
			counter.put(c, counter.get(c) -1);
		}

		return true;
	}
}

ermutationOfStringTest.java

import static org.junit.Assert.*;

import org.junit.Test;


public class PermutationOfStringTest {

	@Test
	public void test() {
		assertEquals(PermutationOfString.solution(null, ""), false);
		assertEquals(PermutationOfString.solution("", null), false);
		assertEquals(PermutationOfString.solution(null, null), false);
		assertEquals(PermutationOfString.solution("", ""), true);
		assertEquals(PermutationOfString.solution("a", ""), false);
		assertEquals(PermutationOfString.solution("", "a"), false);
		assertEquals(PermutationOfString.solution("adeba", "bade"), false);
		assertEquals(PermutationOfString.solution("adeba", "badea"), true);
		assertEquals(PermutationOfString.solution("aabde", "edbaa"), true);
		assertEquals(PermutationOfString.solution("aabde", "edbca"), false);
		assertEquals(PermutationOfString.solution("aabde", "edbba"), false);
	}

}