//https://leetcode.com/problems/combinations/

Combinations.java

//https://leetcode.com/problems/combinations/
// 深度优先搜索框架
public class Solution {
    public static List<List<Integer>> ans = new ArrayList<List<Integer>>();
    
    public static int[] path = new int[100];
    
    public static int K = 0;
    
    public static void robot(int idx, int n, int k){//idx [0, n] 
        if(k == 0){//choose end
            //record ans
            List<Integer> tmp = new ArrayList<Integer>();
            for(int i=K - 1; i >=0 ;i--){
                tmp.add(path[i]);
            }
            ans.add(tmp);
            return;
        }
        for(int i = idx + 1; i<=n; i++){//下一层搜索的范围, 如果单调, 则要比idx大
            path[k - 1] = i;
            robot(i, n, k - 1);
        }
    }
    
    public List<List<Integer>> combine(int n, int k) {
        ans.clear();
        K = k;
        robot(0, n, k);
        return ans;
    }
}