43. Multiply Strings
把两个数反过来算
1 public String multiply(String num1, String num2) { 2 if(num1 == null || num1.length() == 0 || num2 == null || num2.length() == 0) { 3 return ""; 4 } 5 if(num1.equals("0") || num2.equals("0")) { 6 return "0"; 7 } 8 StringBuilder n1 = new StringBuilder(num1); 9 n1 = n1.reverse(); 10 StringBuilder n2 = new StringBuilder(num2); 11 n2 = n2.reverse(); 12 int[] digit = new int[num1.length()+ num2.length() - 1]; 13 for(int i = 0; i < n1.length(); i++) { 14 for(int j = 0; j < n2.length(); j++) { 15 digit[i + j] += ((int)n1.charAt(i)- '0') * ((int)n2.charAt(j) - '0'); 16 } 17 } 18 int carry = 0; 19 StringBuilder res = new StringBuilder(); 20 for(int i = 0 ; i < digit.length; i++) { 21 System.out.println(digit[i]); 22 } 23 for(int i = 0; i < digit.length; i++) { 24 int temp = digit[i] + carry; 25 res.insert(0, temp % 10); 26 carry = temp / 10; 27 } 28 if(carry != 0) { 29 res.insert(0, carry); 30 } 31 int i = 0; 32 while(i < res.length() && res.charAt(i) == '0') { 33 res.deleteCharAt(i); 34 } 35 return res.toString().equals("") ? "": res.toString(); 36 }
ref: https://zhongyinzhang.wordpress.com/2014/03/13/multiply-strings/r