解题思路

回溯算法

python 代码

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        trace = []
        ret = []
        self.backtrace(n,k,1,trace,ret)
        return ret
    
    def backtrace(self,n,k,start,trace,ret):
        if len(trace)==k:
            ret.append(trace[:])
            return

        for i in range(start,n+1):
            trace.append(i)
            self.backtrace(n,k,i+1,trace,ret)
            trace.pop()

s = Solution()
result = s.combine(4,2)
print(result)