c# collections questions

Question

can anybody help me to find out the following questions

1) input a sentance and print the frequency of words repeating in the sentance. use arraylist or hash table

2) create a linked list . add new elements and delete existing elements and traverse through the list

3)write a prgm that accept a postfix expression nd determines its value.

        input specification:
 the input to the prm will consist os a well formed postfix expresion terminated by a newline charector ('\n'). the expression will consist of operands and operators seperated by a single sapace . this possible operators are 

+ Adddition
- subtratoin
* multiplication
/ integer division
% remainder
^ exponentiation

operants will be positive integers

input will not be such tht it will cause an overflow or underflow in an integer variable

output spec

output should be the value of the expression follwed by a newline charector

example

input
10 20+
output
30

input
9 8+3 5 - /
output
-8

input
7 3 2 ^ %
output
8