Leetcode 43. Multiply Strings

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 = mul + pos[p2];

pos[p1] += sum / 10;
pos[p2] = (sum) % 10;
}
}

StringBuilder sb = new StringBuilder();
for(int p : pos) sb.append(p);
return sb.length() == 0 ? "0" : sb.toString();
}
}