77. Combinations
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],
]
Solution: DFS
class Solution(object):
def combine(self, n, k):
"""
:type n: int
:type k: int
:rtype: List[List[int]]
"""
combinations = []
combination = []
def dfs(start, k):
if n - start + 1 < k:
return
if k <= 0:
combinations.append(list(combination))
return
for num in xrange(start, n + 1):
combination.append(num)
dfs(num + 1, k - 1)
combination.pop()
dfs(1, k)
return combinations