【leetcode】77.组合

题目

77.组合

给定两个整数 n 和 k，返回范围 [1, n] 中所有可能的 k 个数的组合。

你可以按 任何顺序 返回答案。

示例 1：

输入：n = 4, k = 2
输出：
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

示例 2：

输入：n = 1, k = 1
输出：[[1]]

题解

1. 回溯

class Solution(object):def combine(self, n, k):""":type n: int:type k: int:rtype: List[List[int]]"""result = []path = []start = 1def backtracking(n, k, path, start, result):if len(path) == k:result.append(path[:])return for i in range(start, n + 1):path.append(i)backtracking(n, k, path, i + 1, result)path.pop()if n == 1:return [[1]]backtracking(n, k, path, start, result)return result

2. 剪枝优化

n + 1 - (k - len(path)) + 1

class Solution(object):def combine(self, n, k):""":type n: int:type k: int:rtype: List[List[int]]"""result = []path = []start = 1def backtracking(n, k, path, start, result):if len(path) == k:result.append(path[:])return for i in range(start, n + 1 - (k - len(path)) + 1):path.append(i)backtracking(n, k, path, i + 1, result)path.pop()if n == 1:return [[1]]backtracking(n, k, path, start, result)return result