public String multiply(String num1, String num2) { int m = num1.length(); int n = num2.length(); int[] vals = new int[m+n]; for(int i = m-1; i>=0; i--){ for(int j = n-1; j>=0; j--){ int prod = (num1.charAt(i)-'0') * (num2.charAt(j )- '0'); int sum = vals[i+j+1]+prod; vals[i+j+1] = sum%10; vals[i+j] += sum/10; } } StringBuilder sb = new StringBuilder(); for(int val: vals){ if(sb.length() ==0 && val == 0) continue; sb.append(val); } return sb.length()==0?"0":sb.toString(); }