【Leetcode】【Medium】Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

解题：

模拟乘法运算，可以完全按照模拟的思路，用num1的每一位乘num2，得到的数组按位保存在结果字符串中，并不断更新。

先把字符串反转，在逻辑上思路会更加清晰，当然不反转也可以。

class Solution {
public:
    string multiply(string num1, string num2) {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        int l1 = num1.size();
        int l2 = num2.size();
        string res(l1 + l2, '0');
        int carry, d;
        
        for (int i = 0; i < l1; ++i) {
            int n1 = num1[i] - '0';
            carry = 0;
            for (int j = 0; j < l2; ++j) {
                int n2 = num2[j] - '0';
                d = n1 * n2 + carry + (res[i+j] - '0');
                carry = d / 10;
                res[i+j] = '0' + (d - carry * 10);
            }
            
            int idx = 0;
            while (carry != 0) {
                d = (res[i+l2+idx] - '0') + carry;
                carry = d / 10;
                res[i+l2] = '0' + (d - carry * 10);
                idx++;
            }
        }
        
        while (!res.empty() && res.back() == '0') 
            res.pop_back();
        if (res.empty()) 
            return "0";
        reverse(res.begin(), res.end());
        return res;
    }
};