★ Describe a $\Theta(n\lg{n})$-time algorithm that, given a set $S$ of $n$ integers and another integer $x$, determines whether or not there exists two elements of $S$ whose sum is exactly $x$.

First we sort $S$. Afterwards, we iterate it and for each element $i$ we perform a binary search to see if there is an element equal to $x - i$. If one is found, the algorithm returns true. Otherwise, it returns false.

We iterate $n$ elements and perform binary search for each on in an $n$-sized array, which leads to $\Theta(n\lg{n})$-time. If we sort first (with merge sort) it will introduce another $\Theta(n\lg{n})$ time, that would change the constant in the final algorithm, but not the asymptotic time.

Here's the pseudocode:

```
PAIR-EXISTS(S, x):
A = MERGE-SORT(S)
for i = 1 to S.length
if BINARY-SEARCH(A, x - S[i]) != NIL
return true
return false
```