Problem

Given a list of numbers, return all possible permutations.

 Notice

You can assume that there is no duplicate numbers in the list.

Example
For nums = [1,2,3], the permutations are:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

思路

同permutation II, 少了重复元素, 不用排序

    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            result.add(new ArrayList<Integer>());
            return result;
        }
        boolean[] visited = new boolean[nums.length];
        helper(nums, result, new ArrayList<Integer>(), visited);
        return result;
    }

    private void helper(int[] nums, List<List<Integer>> result, List<Integer> path, boolean[] visited) {
        for (int i = 0; i < nums.length; i++) {
            if (visited[i]) continue;
            visited[i] = true;
            path.add(nums[i]);
            if (path.size() == nums.length) {
                result.add(new ArrayList<Integer>(path));
            } else {
                helper(nums, result, path, visited);
            }
            path.remove(path.size() - 1);
            visited[i] = false;
        }
    }