模板
def backtrack(nums: List[int], track: List[int], res: List[List[int]]):
for 选择 in 选择列表:
track.append(nums[i])#做选择
backtrack(nums, track, res)#递归
track.pop() # 撤销选择
def backtrack(nums: List[int], track: List[int], res: List[List[int]]):
for 选择 in 选择列表:
track.append(nums[i])#做选择
backtrack(nums, track, res)#递归
track.pop() # 撤销选择