LeetCode 043 Multiply Strings

题目要求：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.

 

分析：

参考网址：http://blog.csdn.net/pickless/article/details/9235907

利用竖式的思想，进行乘法运算。

 

        3    4

*      1    3

——————

       9      12

3     4

——————

3     13     12

 

再处理进位：

3 13 12 - > 3 14 2 -> 4 4 2

 

代码如下：

class Solution {

public:

    string multiply(string num1, string num2) {



        int flag = 1;

        

        //先处理符号

        if (num1[0] == '-' || num2[0] == '-') {

            if (num1[0] == '-') {

                flag *= -1;

                num1 = num1.substr(1, num1.size() - 1);

            }

            if (num2[0] == '-') {

                flag *= -1;

                num2 = num2.substr(1, num2.size() - 1);

            }

        }

        

        int length = num1.size() + num2.size() + 1;

        int s[length];

        

        memset(s, 0, sizeof(int) * length);

        

        int i = 0, temp = 0;

        

        for (i = 0; i < num1.size(); i++) {

            for (int j = 0; j < num2.size(); j++) {

                s[(num1.size() - i - 1) + (num2.size() - j - 1)] += (num1[i] - '0') * (num2[j] - '0');

            }

        }

        

        //进位

        for (i = 0; i < length; i++) {

            s[i + 1] += s[i] / 10;

            s[i] = s[i] % 10;

        }

        

        for (i = 0; i < length / 2; i++) {

            temp = s[i];

            s[i] = s[length - i - 1];

            s[length - i - 1] = temp;

        }

        

        string ans = flag < 0 ? "-" : "";

        for (i = 0, temp = -1; i < length; i++) {

            if (s[i] != 0) {

                temp = 1;

            }

            if (temp > 0) {

                ans += s[i] + '0';

            }

        }

        return ans == "" ? "0" : ans;

    }

};