leetcode-46全排列，使用递归回溯

深度搜索优先，树型结构

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        int len = nums.length;
        List<List<Integer>> list = new ArrayList<>();
        if (len==0){
            return list;
        }
        Deque<Integer> deque = new ArrayDeque<>();
        boolean[] isused = new boolean[len];//标记该点元素是否可用
        method(nums,len,0,deque,isused,list);
        return list;
    }

    private void method(int[] nums, int len, int height, Deque<Integer> deque, boolean[] isused, List<List<Integer>> list) {
        if (height==len){
            list.add(new ArrayList<>(deque));
            return;
        }
        for (int i = 0; i < len; i++) {
            if (isused[i]){
                continue;
            }
            deque.addLast(nums[i]);
            isused[i]=true;
            method(nums,len,height+1,deque,isused,list);
            deque.removeLast();
            isused[i]=false;
        }
    }
}