Leetcode/F家--43. Multiply Strings(Math)

43. Multiply Strings(Math)
Medium 
https://leetcode.com/problems/multiply-strings/

 Company: F家，twitter
思路：  conclude that

 `num1[i] * num2[j]` will be placed at indices `[i + j`, `i + j + 1]` //1st pos: inc, 2nd: sum + mul

using int[] to add up results in double for loop.O(mn)

public class Solution {
    public String multiply(String num1, String num2) {       
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n];
        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');
                int p1 =i+j,p2=i+j+1;
                int sum = pos[p2]+mul;
                pos[p1] += sum/10;
                pos[p2] = sum%10;
            }
        }
        StringBuilder sb = new StringBuilder();
        for(int i=0;i<m+n;i++){ 
            if(!(sb.length() == 0 && pos[i] == 0)) sb.append(pos[i]);//avoid "00"
        }
        return sb.length() == 0 ? "0" : sb.toString();
    }
}