LeetCode46. 全排列

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length < 1) return res;
        dfs(nums, 0 ,res); // 通过交换
//        boolean[] visited = new boolean[nums.length];
//        dfs1(nums,visited,new ArrayList<>() ,res); // 标记数组来处理填过的数
        return res;
    }
    private void dfs1(int[] nums,boolean[] visited,List<Integer> list, List<List<Integer>> res) {
        if (list.size() == nums.length) {
            res.add(new ArrayList<>(list));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (visited[i]) continue;
            visited[i] = true;
            list.add(nums[i]);
            dfs1(nums, visited, list, res);
            visited[i] = false;
            list.remove(list.size() - 1);
        }
    }
    private void dfs(int[] nums, int start, List<List<Integer>> res) {
        if (start == nums.length) {
            List<Integer> list = new ArrayList<>();
            for (int num : nums) {
                list.add(num);
            }
            res.add(list);
            return;
        }
        for (int i = start; i < nums.length; i++) {
            swap(nums, start, i); // 每交换一次就确定一个位置上的元素
            dfs(nums, start + 1, res);
            swap(nums, start, i);
        }
    }
    private void swap(int[] nums, int a, int b) {
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
}