LeetCode: Reverse Words in a String：Evaluate Reverse Polish Notation
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
地址：https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
计算波兰后缀表达式的值，具体做法是利用一个栈，如果遇到数字则把数字进栈，如果遇到操作符就把栈中的数字出栈进行运算（二元操作符出栈两个数字，一元操作符出栈一个数字），最后的结果存在栈中。代码：
1 class Solution {
2 public:
3 int evalRPN(vector<string> &tokens) {
4 int lop, rop;
5 vector<string>::const_iterator cIter = tokens.begin();
6 stack<int> s;
7 for(; cIter != tokens.end(); ++cIter){
8 if ((*cIter).size() == 1 && !isdigit((*cIter)[0])){
9 char op = (*cIter)[0];
10 rop = s.top();
11 s.pop();
12 lop = s.top();
13 s.pop();
14 if ('+' == op){
15 s.push(lop + rop);
16 } else if ('-' == op){
17 s.push(lop - rop);
18 } else if ('*' == op){
19 s.push(lop * rop);
20 } else{
21 s.push(lop / rop);
22 }
23 } else {
24 s.push(atoi((*cIter).c_str()));
25 }
26 }
27 return s.top();
28 }
29 };