七.前后缀分解

238. 除自身以外数组的乘积

class Solution:
    def productExceptSelf(self, nums: List[int]) -> List[int]:
        n = len(nums)
        p = [1] * n; s = 1
        for i in range(1, n):
            p[i] = p[i-1] * nums[i-1]
        for i in range(n - 1, -1, -1):
            p[i] *= s
            s *= nums[i]
        return p

# 前后缀数组
# p前缀数组，s后缀数组
# 因为空间复杂度要求 O(1)，因此结果返回也使用p数组，同时后缀数组存储在变量中
# 时间复杂度：O(N)
# 空间复杂度：O(1)

class Solution:
    def productExceptSelf(self, nums: List[int]) -> List[int]:
        n = len(nums)
        p = [1] * n; s = [1] * n
        res = list()
        for i in range(1, n):
            p[i] = p[i-1] * nums[i-1]
        for i in reversed(range(n - 1)):
            s[i] = s[i+1] * nums[i+1]
        for i in range(n):
            res.append(p[i] * s[i])
        return res