leetcode--Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

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1. public class Solution {  
2.     public List<List<Integer>> combinationSum3(int k, int n) {  
3.         List<List<Integer>> res = new ArrayList<List<Integer>>();  
4.         if(k<1 || n<1) return res;    
5.         helper(res, k, n, 1, 0, new ArrayList<Integer>());  
6.         return res;  
7.     }  
8.       
9.     public void helper(List<List<Integer>> res,int k,int n,int cur_k,int cur_sum,List<Integer> item){  
10.         if(k==0&&cur_sum==n){  
11.             res.add(new ArrayList<Integer>(item));  
12.         }else if(cur_sum>n||cur_k<=0) return;  
13.         for(int i=cur_k;i<=9;i++){  
14.             item.add(i);  
15.             helper(res, k-1, n, i+1, cur_sum+i, item);  
16.             item.remove(item.size()-1);  
17.         }  
18.     }  
19. } 

原文链接http://blog.youkuaiyun.com/crazy__chen/article/details/46581071