Permutations I & II

I

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

public class Solution {

    public ArrayList<ArrayList<Integer>> permute(int[] nums) {

        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();

        if(nums==null||nums.length==0) return res;

        ArrayList<Integer> t = new ArrayList<Integer>();

        t.add(nums[0]);

        res.add(t);

        for(int i=1;i<nums.length;i++){

            res = helper(res, nums[i]);

        }

        return res;

    }

    private ArrayList<ArrayList<Integer>>  helper(ArrayList<ArrayList<Integer>> res, int k){

        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();

        for(int i=0;i<res.size();i++){

            for(int j=0;j<=res.get(i).size();j++){

                ArrayList<Integer> t = new ArrayList<Integer>(res.get(i));

                t.add(j,k);

                list.add(t);

            }

        }

        return list;

    }

}

 

II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

 

题解：加个hashset 去掉duplicate即可

public class Solution {

    public ArrayList<ArrayList<Integer>> permuteUnique(int[] nums) {

        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();

        if(nums==null||nums.length==0) return res;

        HashSet<ArrayList<Integer>> hs = new HashSet<ArrayList<Integer>>();

        ArrayList<Integer> it = new ArrayList<Integer>();

        it.add(nums[0]);

        res.add(it);

        for(int i=1;i<nums.length;i++){

            res = helper(hs, res, nums[i]);

        }

        return res;

    }

    private ArrayList<ArrayList<Integer>> helper(HashSet<ArrayList<Integer>> hs,ArrayList<ArrayList<Integer>> res, int k){

        ArrayList<ArrayList<Integer>> r = new ArrayList<ArrayList<Integer>>();

        for(int i=0;i<res.size();i++){

            for(int j=0;j<=res.get(i).size();j++){

                ArrayList<Integer> t = new ArrayList<Integer>(res.get(i));

                t.add(j,k);

                if(hs.contains(t))

                    continue;

                hs.add(t);

                r.add(t);

            }

        }

        return r;

    }

}