题目: https://leetcode.com/problems/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]]
题意:
找到所有 k 个数字的组合, 由这 k 个数字相加的和为 n
这几个数字只由 1 to 9 ,并且组合是唯一的,保证这个set的数字是升序的
参考:
http://bookshadow.com/weblog/2015/05/24/leetcode-combination-sum-iii/
回溯法:
http://www.cnblogs.com/steven_oyj/archive/2010/05/22/1741376.html
回溯法是一种选优搜索法,按选优条件向前搜索,以达到目标。
但当探索到某一步时,发现原先选择并不优或达不到目标,就退回一步重新选择,
这种走不通就退回再走的技术为回溯法,而满足回溯条件的某个状态的点称为“回溯点”。
Python
class Solution(object): def combinationSum3(self, k, n): """ :type k: int :type n: int :rtype: List[List[int]] """ ans=[] def search(start,cnt,sums,nums): if cnt>k or sums>n: return if cnt==k and sums==n: ans.append(nums) return for x in range(start+1,10): search(x,cnt+1,sums+x,nums+[x]) search(0,0,0,[]) return ans