Given a binary tree, return the preorder traversal of its nodes' values.
Merge branch 'master' of github.com:jinmingmu/codeingInterview
Given a roman numeral, convert it to an integer.
You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?