Given a collection of distinct 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],
  [3,2,1]
]

题意：求一个字符串的全排序。

用递归，dfs来实现。

java代码：

 private List<Integer> temp = new ArrayList<>();

    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> list = new ArrayList<>();
        dfs(list, nums, 0);
        return list;
    }

    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }

    private void dfs(List<List<Integer>> list, int[] nums, int j) {
        if (j == nums.length) {
            for (int num : nums)
                temp.add(num);
            list.add(temp);
            temp = new ArrayList<>();
        }
        for (int i = j; i < nums.length; i++) {
            swap(nums, i, j);
            dfs(list, nums, j+1);
            swap(nums, i, j);
        }
    }