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].

 

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
    	List<List<Integer>> res = new ArrayList<List<Integer>>();
    	if(nums.length==0 || nums==null) {
    		return res;
    	}
    	List<Integer> item = new ArrayList<Integer>();
    	boolean[] visited = new boolean[nums.length];  
        help(nums,res,item,visited);
        return res;
    }

	private void help(int[] nums, List<List<Integer>> res, List<Integer> item,
			boolean[] visited) {
		// TODO Auto-generated method stub
		if (item.size() == nums.length) {
			res.add(new ArrayList<Integer>(item));
		}
		for (int i = 0; i < nums.length; i++) {
			if (!visited[i]) {
				item.add(nums[i]);
				visited[i] = true;
				help(nums,res,item,visited);
				item.remove(item.size()-1);
				visited[i] = false;
			}
		}
	}

}