LeetCode Permutations

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].

采用递归的思路，一次加一个数，每次更新上一次的结果。

public class Solution {

    public List<List<Integer>> permute(int[] num) {
        List<List<Integer>> lastList=new ArrayList<List<Integer>>();
        List<List<Integer>> currList=new ArrayList<List<Integer>>();
            if (num==null) {
                return lastList;
            }
            
            List<Integer> eleList=new ArrayList<>();
            eleList.add(num[0]);
            lastList.add(eleList);
            
            if (num.length==1) {
                return lastList;
            }
            for (int i = 1; i < num.length; i++) {
                for (List<Integer> l : lastList) {
                    currList.addAll(insertList(num[i], l));
                    
                }
                lastList.clear();
                lastList.addAll(currList);
                currList.clear();
        }
            return lastList;
            
    }
    
    private List<List<Integer>> insertList(int num,List<Integer> list) {
            List<List<Integer>> retNew=new ArrayList<List<Integer>>();
            
            for (int i = 0; i <= list.size(); i++) {
                    List<Integer> currlist=new ArrayList<>();
                currlist.addAll(list);
                currlist.add(i, num);
                retNew.add(currlist);
                currlist=null;
            }
            return retNew;
        
    }
}