/* Given two non-negative integers num1 and num2 represented as string,
return the sum of num1 and num2.
Note:
The length of both num1 and num2 is < 5100.
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 addStrings(String num1, String num2) {
// corner case
if(num1 == null || num1.length() == 0)
return num2;
if(num2 == null || num2.length() == 0)
return num1;
// use StringBuilder to store the reversed result.
StringBuilder sb = new StringBuilder();
int index1 = num1.length()-1;
int index2 = num2.length()-1;
int z = 0;
// iterate two String from right to left
while(index1 >= 0 || index2 >= 0){
//empty position will be taken as 0
int x = index1 >= 0 ? num1.charAt(index1) - '0': 0;
int y = index2 >= 0 ? num2.charAt(index2) - '0': 0;
sb.append((x + y + z) % 10);
z = (x + y + z) / 10;
index1--;
index2--;
}
// add an extra digit
if(z == 1)
sb.append("1");
// return reversed StringBuilder
return sb.reverse().toString();
}
}