[Leetcode] combinations

Given two integersNAndK, Return all possible combinationsKNumbers out of 1...N.

For example,
IfN= 4 andK= 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

Search for a forest using DFS. During the search, the child value must be greater than the parent node value for pruning. The forest is in the following shape:

Suppose n = 4, K = 2;

The Code is as follows:

 1 public class Solution { 2     private void comb(int n,int k,int level,List<List<Integer>> answer,List<Integer> result){ 3         if(level == k){ 4             ArrayList<Integer> temp = new ArrayList<Integer>(result); 5             answer.add(temp); 6              7             return; 8         } 9         for(int i = 1;i <= n;i++){10             if(result.isEmpty() || result.get(result.size()-1) < i){11                 result.add(i);12                 comb(n, k, level+1, answer, result);13                 result.remove(result.size()-1);14             }15         }16     }17     public List<List<Integer>> combine(int n, int k) {18         List<List<Integer>> answer = new ArrayList<List<Integer>>();19         List<Integer> result = new ArrayList<Integer>();20         21         comb(n, k, 0, answer, result);22         23         return answer;24     }25 }

Level records the current search level. If K is equal, a combination is found.