43. Multiply Strings(string模拟大数乘法)

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.

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分析：

ac代码：

class Solution {

public:
    string multiply(string num1, string num2) {
        int n1=num1.length(),n2=num2.length(),L=n1+n2,i,j;
        vector<int>ires(L,0);
        string rets(L,'0');

//模拟手算从最后一位开始处理 
        for(i=n1-1;i>=0;i--)
        {
            for(j=n2-1;j>=0;j--)
                ires[i+j+1]+=(num1[i]-'0')*(num2[j]-'0');
        }

//进位 
        for(i=L-1;i>0;i--)
        {
            ires[i-1]+=ires[i]/10;
            ires[i]=ires[i]%10;
        }

//转换成字符串 
        for(i=L-1;i>=0;i--)
        {
            rets[i]=ires[i]+'0';
        }

//前导0
        for(i=0;i<L;i++)
        {
            if(rets[i]!='0')
                break;
        }
        if(i==L)
            return "0";
        return rets.substr(i,L-i);
    }
};