Multiply Strings[LeetCode]

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.

class Solution {public:string multiply(string num1, string num2) {if(num1 == "0" || num2=="0") return "0";int len1 = num1.size();int len2 = num2.size();int flag = 0;int k = 0;string str(len1 + len2, '0');for(int i=len1-1;i>=0;i--){int a = num1[i] - '0';for(int j=len2-1, k=i+len2; j>=0; j--,k--){int b = num2[j] - '0';int unit = a*b %10;int cur_unit = str[k] - '0';str[k] = (cur_unit + unit + flag)%10 + '0';//flag为每次进位的值，待下次计算时加入flag = (a*b - unit)/10 + (cur_unit + unit + flag)/10;}//处理每次循环到num2的最高位后的flagint m = i;while(flag){int cur = str[m] - '0';str[m] = (cur + flag)%10 + '0';flag = (cur + flag)/10;m--;}}int x = 0;//去除str前面的‘0’        while (str[x] == '0')            x++;str = str.substr(x,len1+len2-x);return str;}};