Leetcode 77, Combinations

Posted 2020-08-22 lettuan

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

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

 1 class Solution(object):
 2     def combine(self, n, k):
 3         """
 4         :type n: int
 5         :type k: int
 6         :rtype: List[List[int]]
 7         """
 8         res = []
 9         nums = list(range(1, n+1))
10         self.dfs(nums, k, 0, res, [])
11         
12         return res
13         
14     def dfs(self, nums, k, step, res, line):
15         if len(nums) < k - step:
16             return
17         if step == k:
18             res.append([x for x in line])
19         
20         for i, x in enumerate(nums):
21             line.append(nums[i])
22             self.dfs(nums[i+1:], k, step + 1, res, line)
23             line.pop() 
24             

 

L15-L16  判段一下剩下的可以填的元素个数是不是比剩下的空位少，如果是的话可以提前结束这个branch的搜索。没有这两行的话，程序超时。