Find the Missing Integer

Given an array that contains all of the integers from $1$ to $n$ with exactly one of them missing, we want to find the missing

integer. For example, given the list $1,6,2,5,3,$ the missing integer is $4.$

There is a simple but slow $)$ algorithm $($loop through the array and search for each integer, and return the first one

which isn't there$)$ to solve this problem. Your task is to implement the following function in missing. c$,$ using the HashTable ADT to solve it more efficiently:

You should assume that the array is always well$-$formed, i$.$e$.,$ it satisfies the property described above $($and hence the problem will always have a unique answer$).$