Permutation.java

package algorithm.recursion;

import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;


public class Permutation {

 /*
 TASK
 순열을 구한다.
 */

 @Test
 public void test() {
 List<String> actual = new ArrayList<>();
 actual.add("123");
 actual.add("132");
 actual.add("213");
 actual.add("231");
 actual.add("312");
 actual.add("321");
 assertThat(calcPermutation("123"), is(actual));
 }

 public List<String> calcPermutation(String str) {
 if (str == null) return null;
 return permutation(str, new boolean[str.length()],
 "", new ArrayList<>());
 }

 private List<String> permutation(String str, boolean[] isPick,
 String perm, List<String> result) {
 if (str.length() == perm.length()) {
 result.add(perm);
 return result;
 }

 for (int i = 0; i < str.length(); i++) {
 if (isPick[i]) {
 continue;
 }
 isPick[i] = true;
 permutation(str, isPick, perm + str.charAt(i), result);
 isPick[i] = false;
 }
 return result;
 }
}