[Leetcode] 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.

大数乘法，注意前置0的处理。

class Solution {
public:
    string multiply(string num1, string num2) {
        string num3;
        num3.resize(num1.size() + num2.size());
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        int carry = 0;
        int val;
        int i, j, k;
        for (i = 0; i < num3.size(); ++i) {
            num3[i] = '0';
        }
        for (i = 0; i < num1.size(); ++i) {
            carry = 0;
            for (j = 0; j < num2.size(); ++j) {
                val = (num3[i+j] - '0') + (num1[i] - '0') * (num2[j] - '0') + carry;
                carry = val / 10;
                val %= 10;
                num3[i+j] = '0' + val;
            }
            num3[i + j] = '0' + carry;
        }
        int prezero = 0;
        for (i = num3.size() - 1; i > 0; --i) {
            if (num3[i] == '0') 
                prezero++;
            else 
                break;
        }
        num3.resize(num3.size() - prezero);
        reverse(num3.begin(), num3.end());
        return num3;
    }
};