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;
    }
}