43. Multiply Strings (JAVA)

Given two non-negative integers num1 and num2represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"

Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"

Output: "56088"

Note:

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

需要考虑乘积的长度，以及乘积的每一位对应的是乘数中哪两个数字的乘积。

注意字符串首位出现0的情况，通常情况下至多首位为0，除了一个特殊情况乘数中有0，这样会造成String中多位为0，所以在开头要排除这个可能。

注意JAVA比较字符串相等必须用equals而不能用==；字符转换成int，除了-'0'，还需要强制转换(char)；char转成String，也要显示转换，使用String.valueOf

class Solution {

    public String multiply(String num1, String num2) {

        if(num1.equals("0")|| num2.equals("0")) return "0";

        int[] product = new int[num1.length()+num2.length()]; //array to save the product

        int m1;

        int m2;

        int p;

        for(int i = product.length-1; i >= 0; i--){ //iterate each bit of the product

            for(int j = num2.length()-1; j>=0; j--){ //iterate each bit of multiplier2

                if(i-j-1 < 0) continue;

                if(i-j-1 >= num1.length()) break;

                m2 = num2.charAt(j)-'0';

                m1 = num1.charAt(i-j-1)-'0';

                p = m1*m2;

                product[i] += p;

            }

        }

        //calculate carry

        for(int i = product.length-1; i > 0; i--){

            if(product[i]<10) continue;

            product[i-1] += (product[i]/10);

            product[i] %= 10;

        }

        //transfrom integer to string

        String result;

        if(product[0] != 0) {

            result = String.valueOf((char) (product[0] + '0'));

        }

        else result = "";

        for(int i = 1; i < product.length; i++){

            result += String.valueOf((char) (product[i] + '0'));

        }

        return result;

    }

}

巴特西

43. Multiply Strings (JAVA)

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