[leetCode]剑指 Offer 66. 构建乘积数组

解法

把A数组分为两部分：

• A[0,...,i-1]
• A[i+1,...,n-1]
  数组B可以用一个矩阵创建：
  
  另C[i]为矩阵第i行左半部分的乘积，D[i]为矩阵第i行右半部分的乘积。C[i]可以自上而下计算：C[i] = C[i-1] * A[i-1]，D[i]可以自下而上计算:D[i] = D[i+1]*A[i+1]，所以B[i] = C[i] * D[i]

class Solution {
    public int[] constructArr(int[] a) {
        if (a == null || a.length == 0) return new int[]{};
        int[] b = new int[a.length];
        b[0] = 1;
        for (int i = 1; i < a.length; i++) {
            b[i] = b[i-1] * a[i-1];
        }
        int temp = 1;
        for (int i = a.length - 2; i >= 0; i--) {
            temp *= a[i+1];
            b[i] *= temp;
        }
        return b;
    }
}