'''

Find the missing number in a sequence

Find the only missing integer in a sequence,

all numbers are integers and they're smaller or equal to N+1 (N is length of the array).

Input: [2, 1, 4]

Output: 3

=========================================

Searching for 1 unknown, math problem.

Use the sum formula for the first N numbers to compute the whole sum of the sequence.

After that sum all elements from the array, and when you subtract those 2 numbers, you'll get the missing number.

Sum formula = N*(N+1)/2

Time Complexity: O(N)

Space Complexity: O(1)

'''

############

# Solution #

############

def missing_number(nums):

s = sum(nums)

n = len(nums) + 1

# sum formula (sum of the first n numbers) = (N*(N+1))/2

return n * (n + 1) // 2 - s

###########

# Testing #

###########

# Test 1

# Correct result => 4

print(missing_number([2, 3, 1]))

# Test 2

# Correct result => 3

print(missing_number([2, 1, 4]))