77. Combinations

经典combinations。
用模版

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        
        def dfs(n, k, progress, path, result):
            if len(path) == k:
                result.append(path[:])
                return
            for i in range(progress, n):
                path.append(i+1)
                dfs(n, k, i+1, path, result)
                path.pop()
                
        result=[]
        dfs(n, k, 0, [], result)
        return result
        

或者不喜欢参数传递的版本

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:        
        def dfs(progress):
            if len(path) == k:
                result.append(path[:])
                return
            for i in range(progress, n):
                path.append(i+1)
                dfs(i+1)
                path.pop()
        path = []
        result=[]
        dfs(0)
        return result