fix mistakes in solution to Exercise 10.2-2.

All the three procedures of stack have been modified.

Modified are the following 2 files:

	10.2-2.pdf
	10.2-2.tm

Thank you!
							Lawrence X. Amlord
							  May 18th, 2014
							  Xi'an, China
add solution to Exercise 10.2-3.

Included are the following 2 files:

	10.2-3.pdf
	10.2-3.tm

Thank you!
							Lawrence X. Amlord
							  May 18th, 2014
							  Xi'an, China