# LeetCode 371. Sum of Two Integers

## The Problem

Link to original problem on Leetcode.

Given two integers `a`

and `b`

, return *the sum of the two integers without using the operators* `+`

*and* `-`

.

## Examples

Example 1:

```
Input: a = 1, b = 2
Output: 3
```

Example 2:

```
Input: a = 2, b = 3
Output: 5
```

## Constraints

- -1000 <=
`a, b`

<= 1000

## My Solution

To add two numbers without `+`

or `-`

, we’ll use binary. I mainly write JavaScript for web application, which means I use bitwise operations approximately never. But hey, that’s leetcode!

Anyway, bitwise operations include:

`&`

, AND, which returns`1`

if two compared bits are both`1`

`|`

, OR, which returns`1`

if either compared bit is`1`

`~`

, NOT, which returns the opposite of a given bit`^`

, XOR (exclusive or), which returns`1`

if two bits are`0`

and`1`

, but returns`0`

if both bits are either`0`

or`1`

`<<`

shift bits to the left, e.g.,`0110`

to`1100`

`>>`

shift bits to the right, e.g.,`0110`

to`0011`

So, how can we use this information to find the sum of integers?

First, let’s think of an example pair of integers: `7`

and `4`

. In binary, they can be represented as `0111`

and `0100`

. Adding in binary works the same as adding in base ten:

```
0111 => 7
+ 0100 => 4
------
1011 => 11
```

From this simple example, we can see that for each digit, a `0`

and a `1`

will have a `1`

in the result. Two `0`

’s result in a `0`

, and two `1`

’s result in a `0`

and a carried `1`

to the next place.

This means two things:

- We can find the value for a given digit with XOR
- We need a way to carry over the
`1`

when there are two`1`

’s being added

We can figure out where we’ll need carried digits with AND. Then to get them into the right position, we’ll have to shift them left. In our example:

```
0111
& 0100
------
0100 <= the 4's digit is where a 1 will need to carry over
0100
<< 1
------
1000 <= Shifting left by one bit, the carried bit is in the right place
```

Now that we know were our carried digits will go, we can go ahead and find the the digits that should come out to `1`

with XOR.

```
0111
^ 0100
------
0011
```

We’re close, but remember, we can’t use `+`

here! We can’t say `1000 + 0011 = 1011`

and be done with it. Instead, we’ll change the values of our original variables with our new results.

First, find the digits to carry:

```
0011 => 7 ^ 4
& 1000 => (7 & 4) << 1
------
0000 <= nothing left to carry over!
```

Now we find the digits that will equal `1`

using XOR:

```
0011 => 7 ^ 4
^ 1000 => (7 & 4) << 1
------
1011 => 11, our answer!
```

Time to translate this to code. We’re going to want a variable to keep track of digits to carry, and a loop to continue executing these steps until we’ve got our answer. Note that in our example, our carried over digits ended up at `0`

—we can use that as a conditional for our loop to know when to stop.

In our function, we’re passed to integers, `a`

and `b`

. We can mutate them to keep track of our intermediate steps. Let’s store the XOR’ed values (and ultimately, our answer) in `a`

, and our carried over values in `b`

. We need to have a third variable to store the carried values, because we can’t find out what they should be from `a`

and `b`

if we’ve already mutated `a`

to equal `a ^ b`

.

```
const getSum = (a, b) => {
let carry;
// When there are no digits left to carry, b will
// equal 0 and a will equal our answer.
while (b !== 0) {
// Store the digits we'll need to carry over.
carry = a & b;
// Find the digits that will equal 1 in binary,
// then mutate a to equal the result.
a = a ^ b;
// Mutate b to equal the carried digits, shifted
// once to the left.
b = carry << 1;
}
return a;
};
```

You can also write this code more pithily with recursion like so:

```
const getSum = (a, b) => {
return b ? getSum(a ^ b, (a & b) << 1) : a;
};
```