python3

javascript

java

Missing Number

Given an array **arr** containing n distinct numbers in the range **[0, n]**, return the only number in the range that is missing from the array.

### Input Format
First parameter - number `n`

Second Parameter - array of integers `arr` of size `n`

### Output Format
Return the number

### Example 1
```txt
Input: 
   3
   3 0 1
Output: 
   2
Explanation: 
    n = 3 since there are 3 numbers, so all numbers are in the range [0,3]. 2 is the missing number in the range since it does not appear in arr.
```

### Example 2
```txt
Input: 
   2
   0 1
Output: 
   2
Explanation: 
   n = 2 since there are 2 numbers, so all numbers are in the range [0,2]. 2 is the missing number in the range since it does not appear in arr.
```

#### Constraints
- 1 <= n <= 10^4^
- 0 <= arr[i] <= n
- All the numbers of **arr** are **unique**.
- Expected Time complexity: O(n)
- Expected Space Complexity: O(1)


int solve(int n, vector<int> arr){
//CODE HERE 
}

static int solve(int n, int[] arr){
//CODE HERE 
}