leetcode: Multiply Strings

http://oj.leetcode.com/problems/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.

思路：

这是一道经常出现的面试题。竖式乘法怎么做的照搬就可以，注意看代码，用一个string搞定。

class Solution {
public:
    string multiply(string num1, string num2) {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        
        if (("0" == num1) || ("0" == num2)) {
            return "0";
        }
        
        string result = "";
        int i, j;
        int flag = 0, steps = 0;
        
        for (int i = 0; i < num1.length(); ++i) {
            int position = steps;
            
            for (int j = 0; j < num2.length(); ++j) {
                int v = (num1[i] - '0') * (num2[j] - '0') + flag;

                if (position < result.length()) {
                    v += result[position] - '0';
                    result[position] = (v % 10) + '0';
                }
                else {
                    result.append(1, (v % 10) + '0');
                }
                
                flag = v / 10;
                ++position;
            }
            
            if (flag > 0) {
                result.append(1, flag + '0');
            }
            
            flag = 0;
            ++steps;
        }
        
        reverse(result.begin(), result.end());
        
        return result;
    }
};