摘要：
Evaluate the value of an arithmetic expression inReverse Polish Notation.Valid operators are+,-,*,/. Each operand may be an integer or another express... 阅读全文
摘要：
Given an input string, reverse the string word by word.For example,Given s = "the sky is blue",return "blue is sky the".思路:就是对首尾空格和中间连续空格的处理。为了方便处理，在s... 阅读全文
摘要：
Given a binary tree, determine if it is a valid binary search tree (BST).Assume a BST is defined as follows:The left subtree of a node contains only n... 阅读全文