leetcode216. Combination Sum III

题目描述（难度）：

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.
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]]


Tags： Array，Backtracking
Similar Problems： (M) Combination Sum

分析：只有数字1--9可以使用，每个组合里每个数字都只能出现1次

代码实现：

class Solution(object):
    def combinationSum3(self, k, n):
        """
        :type k: int
        :type n: int
        :rtype: List[List[int]]
        """
        candidates = [i for i in range(1, 10)]
        result = []
        
        def dfs(start, tmpRes, n, k):
            if n==0 and k==0:
                result.append(tmpRes[:])
                return
            
            for i in range(start, len(candidates)):
                #选择起点
                if n-candidates[i]<0 or k==0:#剪枝
                    break
                tmpRes.append(candidates[i])
                dfs(i+1, tmpRes, n-candidates[i], k-1)
                tmpRes.pop()#还原递归现场
                
        dfs(0, [], n, k)
        return result