第77题. 组合
需要注意剪枝细节
纵向代表递归,横行代表取数
1、回溯
class Solution:
def combine(self, n: int, k: int) -> List[List[int]]:
res = []
self.backtrack(n, k, 1, [], res)
return res
def backtrack(self, n, k, start, path, res):
if len(path) == k:
res.append(path[:])
return
for i in range(start, n+1): # 代表取数
path.append(i)
self.backtrack(n, k, i+1, path, res)
path.pop()
2、回溯优化
class Solution:
def combine(self, n: int, k: int) -> List[List[int]]:
res = []
self.backtrack(n, k, 1, [], res)
return res
def backtrack(self, n, k, start, path, res):
if len(path) == k:
res.append(path[:])
return
for i in range(start, n-(k-len(path))+2):
path.append(i)
self.backtrack(n, k, i+1, path, res)
path.pop()