LeetCode - Valid Parentheses

Reduction problem in compiling principle.

Data Structure: stack

STL - stack:

　　http://www.cplusplus.com/reference/stack/stack/

　　http://sunzhongfang22.blog.163.com/blog/static/33856090201152451347642/

Solver:

　　1. when encounters a pre-bracket, push it to the stack;

　　2. when encounters a post-bracket, pop the stack and compare with it.

Attention to some special inputs.