43. Multiply Strings

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.

class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n]; // 0是最高位
       
        for(int i = m - 1; i >= 0; i--) {
            for(int j = n - 1; j >= 0; j--) {
                int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0'); 
                // p1代表进位， p2表示本位。
                int p1 = i + j, p2 = i + j + 1;
                // sum 就是本位的乘积加上本位已有的值。
                int sum = mul + pos[p2];
                // 进位就是除以10的余数
                pos[p1] += sum / 10;
                // 本位就是剩下的个位数。
                pos[p2] = (sum) % 10;
            }
        }  
        
        StringBuilder sb = new StringBuilder();
        for(int p : pos) 
            if(!(sb.length() == 0 && p == 0)) 
                sb.append(p);
        return sb.length() == 0 ? "0" : sb.toString();
    }
}

class Solution {
    public String multiply(String num1, String num2) {
        int firstLength = num1.length();
        int secondLength = num2.length();
        StringBuilder sb = new StringBuilder();
        int[] resArr = new int[firstLength + secondLength];
        for(int i= firstLength-1; i>=0; i-- ){
            for(int j= secondLength-1; j>=0; j-- ){
                int temp = (num1.charAt(i)-'0')*(num2.charAt(j)-'0');
                temp += resArr[i+j+1];
                resArr[i+j+1] = temp%10;
                resArr[i+j] += temp/10;
            }
        }
        boolean nonZeroFound = false;
        for(int i : resArr){
            if(i != 0) nonZeroFound = true;
            if(nonZeroFound) sb.append(i);
        }
        if(!nonZeroFound) sb.append("0");
        return sb.toString();
    }
}