Leetcode: Sword finger offer 66 Build product array [application of pre and post infix]

analysis

Because you can't use a division sign
So we should consider saving the front and back infix
And make a special judgment , Other normal calculations are sufficient
Be careful None and 【】

ac code

class Solution:
    def constructArr(self, a: List[int]) -> List[int]:
        if not a:
            return []
        n = len(a)
        b = a[::-1]
        preMul, postMul = [0] * n, [0] * n

        preMul[0] = a[0]
        for i in range(1, n):
            preMul[i] = preMul[i - 1] * a[i]
        
        postMul[0] = b[0]
        for i in range(1, n):
            postMul[i] = postMul[i - 1] * b[i]
        
        ans = []
        for i in range(n):
            if i == 0:
                ans.append(postMul[n - 2])
            elif i == n - 1:
                ans.append(preMul[n - 2])
            else:
                ans.append(preMul[i - 1] * postMul[n - i - 2])
        
        return ans

summary

Anteroposterior accretion