[leetcode]Permutations

有是一道组合排列题目。继续用DFS。不过这次在LeetCode网页里直接写，更接近面试的纸写。

public class Solution {
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
        int len = num.length;
        
        if (len == 0) return ans;
        
        boolean[] vis = new boolean[len];
        int[] kase = new int[len];
        
        
        dfs(ans, 0, len, num, kase, vis);
        
        return ans;
        
    }
    
    public void dfs(ArrayList<ArrayList<Integer>> ans, int k, int n, int[] num, int[] kase, boolean[] vis) {
        if (k == n) {
            ArrayList<Integer> arr = new ArrayList<Integer>();
            for (int i = 0; i < kase.length; i++) {
                arr.add(kase[i]);
            }
            ans.add(arr);
            return;
        }
        else {
            for (int i = 0; i < num.length; i++) {
                if (!vis[i]) {
                    kase[k] = num[i];
                    vis[i] = true;
                    dfs(ans, k+1, n, num, kase, vis);
                    vis[i] = false;
                }
            }
        }
    }
}