46. Permutations.py

class Solution:
    def permute(self, nums: List[int]) -> List[List[int]]:
        ans = []
        # base
        if len(nums)==1:
            return [nums[:]] 
        
        for i in range(len(nums)): # counter
            tmp=nums.pop(0)
            res=self.permute(nums)
            for j in res:
                j.append(tmp)
            ans.extend(res)     
            nums.append(tmp) 
        return ans