PermutationsII.java

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

/**
 * Created by cpacm on 2017/6/6.
 */
public class PermutationsII {

    public static void main(String[] args) {
        System.out.println(permuteUnique(new int[]{1, 2, 1, 4, 18}));
    }

    public static List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length == 0) return res;
        boolean[] used = new boolean[nums.length];
        List<Integer> list = new ArrayList<>();
        Arrays.sort(nums);
        unique(nums, used, list, res);
        return res;
    }

    public static void unique(int[] nums, boolean[] used, List<Integer> list, List<List<Integer>> res) {
        if (list.size() == nums.length) {
            res.add(new ArrayList<>(list));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (used[i]) continue;
            if (i > 0 && nums[i - 1] == nums[i] && !used[i - 1]) continue;
            used[i] = true;
            list.add(nums[i]);
            unique(nums, used, list, res);
            used[i] = false;
            list.remove(list.size() - 1);
        }
    }
}