371. Sum of Two Integers

Today I soled the "Sum of Two Integers"

 

Description:

https://leetcode.com/problems/sum-of-two-integers/description/

This question ask us how to get sum of the two integers without using sign "+" and "-."

 

In example 2, 

a is 2 => 10

b is 3 => 11

 

a+b:

   10

+ 11

-----

101 (which is 5)

 

In binary number, 1 + 1 makes the carry "1" to the next slot.

For carry "1", we are gonna use &

 

a & b:

   10

& 11

-----

    100(so move to left <<) then 100

 

a XOR b:

   10

^ 11

-----

   01

 

Now see the result, 

  100

^ 01

----- 

  101 (which is "5")

 

Solution:

 

In Python, we need to constraints the bit range, because it uses large range of value. 

shorter_bit = 0xffffffff shorten the value as 32-bits. 

 

while b is more than 0, (b could be negative integer!) we XOR carry and value. 

Within the loop, we know we have a carry and a XOR b. 

We are gonna XOR again to a XOR b with carry.

 

We are gonna return a, but a should be constraints if b is a negative number.