Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center). For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
    1
   / \
  2   2
...
			

150. Evaluate Reverse Polish Notation

  Evaluate the value of an arithmetic expression in Reverse Polish Notation. Valid operators are + , - , * and /. Each operand may be an integer or another expression. Some examples: ...