46. Permutations——本质和树DFS遍历无异 fun: for i in nums fun(i)

Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

class Solution(object):
    def permute(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        ans = []
        self.permute_helper(nums, 0, ans)
        return ans

    def permute_helper(self, nums, start, ans):
        if start == len(nums):
            ans.append(list(nums))
            return
        for i in range(start, len(nums)):
            nums[i], nums[start] = nums[start], nums[i]
            self.permute_helper(nums, start+1, ans)
            nums[i], nums[start] = nums[start], nums[i]