Use two stacks. Every time you visit a site, push its address in stack1. When you press back, pop from stack1 and also push in stack2. When user presses forward, pop from stack2 and also push in stack1.

May be Stack , any one please correct me if I am wrong.

This can be implemented by using two different stacks, one for back and one for forward.

Doubly linkedList

two stacks. When you visit a site, push its address into stack1 and clear stack2; When you press back, pop the address from stack1 and push the same address into stack2; When you press forward, pop the address from stack2 and push the same address into stack1.

Command Pattern