Topic links: https://leetcode-cn.com/problems/combination-sum-iii/
Subject description:
And the sum of all to find a combination of the number n of k. Composition may only contain 1-- 9 positive integer, and repeatable number does not exist in each combination.
Description:
- All numbers are positive integers.
- Solution Set can not contain duplicate combinations thereof.
Example:
Example 1:
输入: k = 3, n = 7
输出: [[1,2,4]]Example 2:
输入: k = 3, n = 9
输出: [[1,2,6], [1,3,5], [2,3,4]]Ideas:
A thought: library functions
class Solution:
def combinationSum3(self, k: int, n: int) -> List[List[int]]:
return [item for item in itertools.combinations(range(1, 10), k) if sum(item) == n]Thinking two: backtracking algorithm
class Solution:
def combinationSum3(self, k: int, n: int) -> List[List[int]]:
res = []
def helper(k, start, n, tmp):
if k == 0:
if n == 0:
res.append(tmp)
return
for i in range(start, 10):
if n - i < 0 :break
helper(k - 1, i + 1, n - i, tmp + [i])
helper(k, 1, n, [])
return res