全排列(回溯法)

class Solution {
     List<List<Integer>> res = new LinkedList<>();
    List<Integer> list = new LinkedList<>();
    void  backtrack(int[] nums, LinkedList<Integer> track) {
        // 触发结束条件
        if (track.size() == nums.length) {
            res.add(new LinkedList(track));
            return;
        }

        for (int i = 0; i < nums.length; i++) {
            // 排除不合法的选择
            if (track.contains(nums[i]))
                continue;
            // 做选择
            track.add(nums[i]);
            // 进入下一层决策树
            backtrack(nums, track);
            // 取消选择
            track.removeLast();
        }
    }
    public List<List<Integer>> permute(int[] nums) {
    backtrack(nums,(LinkedList<Integer>) list);
    return res;
    }
}