Leetcode:43. Multiply Strings

Description

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2.

Note:

• The length of both num1 and num2 is < 110.
• Both num1 and num2 contains only digits 0-9.
• Both num1 and num2 does not contain any leading zero.
• You must not use any built-in BigInteger library or convert the inputs to integer directly.

思路

• 按照乘法的过程来，中间主要考虑到进位问题

代码

class Solution {
public:
    string multiply(string num1, string num2) {
        int len1 = num1.size();
        int len2 = num2.size();
        string str(len1 + len2 + 1, '0');

        int count = 0;
        int mul = 0;
        int start = 0;
        int i = 0;
        for (i = len2 - 1; i >= 0; --i){
            for (int j = len1 - 1; j >= 0; --j){
                mul = (num2[i] - '0') * (num1[j] - '0') + (str[start + len1 - 1 - j] - '0') + count;
                count = mul / 10;
                str[start + len1 - 1 - j] = '0' + (mul % 10);
            }

            if (count > 0){
                str[start + len1] = '0' + count;
                count = 0;
            }

            start++;
        }

        string tmp;
        for (i = str.size() - 1; i >= 0; --i){
            if (str[i] != '0')  break;
        
        for (int j = i; j >= 0; --j)
            tmp += str[j];

        if (tmp.empty()) return "0";
        return tmp;
    }
};

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