☆打卡算法☆LeetCode 43、字符串相乘 算法解析

示例 1：
输入: num1 = "2", num2 = "3"
输出: "6"

示例 2：
输入: num1 = "123", num2 = "456"
输出: "56088"

public class Solution {
    public string Multiply(string num1, string num2) {
        StringBuilder sum = new StringBuilder(new string('0', num1.Length + num2.Length));
        for (int i = num1.Length - 1; i >= 0; i--)
        {
            for (int j = num2.Length - 1; j >= 0; j--)
            {
              int next = sum[i + j + 1] - '0' + (num1[i] - '0') * (num2[j] - '0');
              sum[i + j + 1] = (char)(next % 10 + '0');
              sum[i + j] += (char)(next / 10);
            }
        }
        string res = sum.ToString().TrimStart('0');
        return res.Length == 0 ? "0" : res;
    }
}