用python写一个回溯算法

以下是一个用Python编写的回溯算法的示例：

def backtrack(candidates, target):
result = []
path = []

def dfs(start):
if sum(path) == target:
result.append(path[:])
return

for i in range(start, len(candidates)):
if sum(path) + candidates[i] > target:
break

path.append(candidates[i])
dfs(i)
path.pop()

dfs(0)
return result

candidates = [2, 3, 6, 7]
target = 7
print(backtrack(candidates, target))

输出：
[[2, 2, 3], [7]]