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.
• Converting the input string to integer is NOT allowed.
• You should NOT use internal library such as BigInteger.

分析： 模拟乘法，注意下细节和进位

class Solution {
public:
    string multiply(string num1, string num2) {
        int size1 = num1.size(), size2 = num2.size();
        if(size1==0 || size2==0) return 0;
        vector<int> res(size1+size2,0);
        for(int i =0; i< size1; i++ ){
           int carry = 0;
           int n1 = (int)(num1[size1-1-i]-'0');
           for(int j =0; j< size2; j++){
               int n2 = (int)(num2[size2-1-j]-'0');
               int sum = (n1*n2+carry+res[i+j]);
               carry = sum/10;
               res[i+j] = sum%10;
           }
           res[i+size2] += carry==0? 0: carry;
        }
        // for(auto t: res)
        //     cout << t<<" ";
        int start =size1+size2-1;
        while(start>=0 && res[start]==0) start--;
        if(start==-1) return "0";
        string s="";
        for(;start>=0; start--)
            s+= (char)(res[start]+'0');
        return s;
            
           
    }
};