LintCode 369 [Convert Expression to Polish Notation]
原题给定一个表达式字符串数组,返回该表达式的波兰表达式。(即去掉括号)对于[(5−6)*7]的表达式(该表达式可表示为["(","5","−","6",")","","7"]),其对应的波兰表达式为[-567](其返回的数值为["*","−","5","6","7"])。解题思路首先建立表达式树,如题[ExpressionTreeBuild]PolishNotation即表达式树前序遍历的结果完整