[leetcode] 77. Combinations @ python

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原题

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

Example:

Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

解法

DFS+回朔法. DFS的关键是找到回溯的条件, 当k= 0时, 表示我们已经取完数字, 此时将path加到结果中, 然后加限制条件: nums的长度需要大于等于K才能取完数字, 然后遍历nums, 递归寻找结果.

Time: O(n)
Space: O(1)

代码

class Solution(object):
    def combine(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[List[int]]
        """
        def dfs(nums, k, path, res):
            if k == 0:
                res.append(path)
                return
            if len(nums) >= k:
                for i in range(len(nums)+1-k):
                    dfs(nums[i+1:], k-1, path + [nums[i]], res)
                    
        res = []
        nums = list(range(1, n+1))
        dfs(nums, k, [], res)
        return res