動機

由左到右與由右到左去組合!!

Problem

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].

The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

You must write an algorithm that runs in O(n) time and without using the division operation.

 

Example 1:

Input: nums = [1,2,3,4]Output: [24,12,8,6]

Example 2:

Input: nums = [-1,1,0,-3,3]Output: [0,0,9,0,0]

 

Constraints:

  • 2 <= nums.length <= 105
  • -30 <= nums[i] <= 30
  • The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

 

Follow up: Can you solve the problem in O(1) extra space complexity? (The output array does not count as extra space for space complexity analysis.)

Sol

由左到右與由右到左，之後兩個放在一起看，再看我們的目標，就會知道怎麼走了

話說follow-up有說用常量記憶體來做，其實就是把下面的ret用成l2r就好

class Solution:
    def productExceptSelf(self, nums: List[int]) -> List[int]:
        l2r = [nums[0]]
        for i in range(1,len(nums)):
            l2r.append(l2r[i-1]*nums[i])
        for i in range(len(nums)-2,0,-1):
            nums[i] = nums[i+1]*nums[i]
        r2l = nums
        # 1       1,2    1,2,3 1,2,3,4
        # 1,2,3,4 2,3,4  3,4   4
        # 2,3,4   1,3,4  1,2,4 1,2,3
        
        ret = [r2l[1]]
        for i in range(len(nums)-2):
            ret.append(l2r[i]*r2l[i+2])
        ret.append(l2r[-2])
        return ret