Leetcode 43. 字符串相乘

计算步骤：
123 *456
A[0]=3*6=18 A[1] =2*6+3*5=27 A[2] =2*5+3*4+6*1=28
A[3]=1*5+4*2=13 A[4]=1*4=4 A[5]=A[6]=…=0
A[0]=8 A[1]=27+1=28
A[1]=8 A[2]=28+2=30
A[2]=0 A[3]=13+3=16
A[3]=6 A[4]=4+1=5
A[4]=5 A[5]=A[6]=…=0
—>A[4]A[3]A[2]A[1]A[0]=56088

class Solution {
public:
    string multiply(string num1, string num2) {
        int A[300] = {}, high;
        for (int i = 0; i < num1.size(); ++i)
            for (int j = 0; j < num2.size(); ++j)
                A[num1.size() + num2.size() - i - j - 2] += (num1[i] - '0')*(num2[j] - '0');
        string ans;
        for (int i = 0; i < 250; ++i) {
            A[i + 1] += A[i] / 10, A[i] %= 10;
            if (A[i]) high = i;
        }
        for (int i = high; i >= 0; --i)
            ans += (A[i] + '0');
        if (ans.empty()) ans = "0";
        return ans;
    }
};