Permutations

Given a collection of 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], and[3,2,1].

思路：经典的backtracking. 不要记代码，弄清楚原理，传递到下一层的是什么信息，然后

choosen

explore

unchoosen

要记录choosen的状态，所以要传递一个visited boolean array；

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> lists = new ArrayList<List<Integer>>();
        List<Integer> list = new ArrayList<>();
        int n = nums.length;
        boolean[] visited = new boolean[n];
        dfs(lists, list, nums, visited);
        return lists;
    }
    
    private void dfs(List<List<Integer>> lists, List<Integer> list, int[] nums,  boolean[] visited) {
        if(list.size() == nums.length) {
            lists.add(new ArrayList<Integer>(list));
            return;
        }
        for(int i = 0; i < nums.length; i++) {
            if(!visited[i]) {
                list.add(nums[i]);
                visited[i] = true;
                dfs(lists, list, nums, visited);
                visited[i] = false;
                list.remove(list.size() - 1);
            }
        }
    }
}