Easy

## Question

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Notice
1) The length of both num1 and num2 is < 5100.
2) Both num1 and num2 contains only digits 0-9.
3) Both num1 and num2 does not contain any leading zero.
4) You must not use any built-in BigInteger library or convert the inputs to integer directly.

Example
Given num1 = "123", num2 = "45"
return "168"

## Review

Since input are all numbers, we can use (char - ‘0’) to get int value of char

## Solution

#### Java (using (char - ‘0’))

``````public class Solution {
/**
* @param num1 a non-negative integers
* @param num2 a non-negative integers
* @return return sum of num1 and num2
*/
public String addStrings(String num1, String num2) {
if (num1 == null || num1.length() == 0) {
return num2;
}
if (num2 == null || num2.length() == 0) {
return num1;
}
int index1 = num1.length() - 1;
int index2 = num2.length() - 1;
int carry = 0;
StringBuilder sb = new StringBuilder();
while (index1 >= 0 || index2 >= 0) {
if (index1 >= 0) {
carry = carry + (num1.charAt(index1) - '0');
}
if (index2 >= 0) {
carry = carry + (num2.charAt(index2) - '0');
}
sb.insert(0, carry % 10);
carry = carry / 10;
--index1;
--index2;
}
if (carry > 0) {
sb.insert(0, carry);
}
return sb.toString();
}
}
``````

#### Java (first solution)

``````public class Solution {
/**
* @param num1 a non-negative integers
* @param num2 a non-negative integers
* @return return sum of num1 and num2
*/
public String addStrings(String num1, String num2) {
if (num1 == null && num2 == null) {
return null;
}
if (num1 == null) {
return num2;
}
if (num2 == null) {
return num1;
}
StringBuilder sb = new StringBuilder();
int index1 = num1.length() - 1;
int index2 = num2.length() - 1;
int carry = 0;
int value = 0;
while (index1 >= 0 || index2 >= 0) {
if (index1 >= 0 && index2 >=0) {
value = carry + Character.getNumericValue(num1.charAt(index1))
+ Character.getNumericValue(num2.charAt(index2));
} else if (index1 >= 0 && index2 < 0) {
value = carry + Character.getNumericValue(num1.charAt(index1));
} else if (index1 < 0 && index2 >= 0) {
value = carry + Character.getNumericValue(num2.charAt(index2));
} else {
// noop
}
carry = value / 10;
sb.insert(0, value % 10);
--index1;
--index2;
}
if (carry > 0) {
sb.insert(0, carry);
}
return sb.toString();
}
}
``````