Let S be a stack of positive integers and let x be an integer variable. Write C++ code fragments to perform each of the following operations: 1. Set x to the top element of S and leave the top element of S unchanged. If S is empty, set x to -1. 2. Set x to the third element from the top in S, provided that S contains at least three integers. If not, set x to -1. Leave S unchanged. 3. Set x to the bottom element of S (or to -1 if S is empty) and leave S unchanged. 4. Delete all occurrences of x from S, leaving the other elements of S in the same order.