LeetCode: Reverse Words in a String：Evaluate Reverse Polish Notation

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：

 class Solution {
 public:
     int evalRPN(vector<string> &tokens) {
         int lop, rop;
         vector<string>::const_iterator cIter = tokens.begin();
         stack<int> s;
         for(; cIter != tokens.end(); ++cIter){
             if ((*cIter).size() ==  && !isdigit((*cIter)[])){
                 char op = (*cIter)[];
                 rop = s.top();
                 s.pop();
                 lop = s.top();
                 s.pop();
                 if ('+' == op){
                     s.push(lop + rop);
                 } else if ('-' == op){
                     s.push(lop - rop);
                 } else if ('*' == op){
                     s.push(lop * rop);
                 } else{
                     s.push(lop / rop);
                 }
             } else {
                 s.push(atoi((*cIter).c_str()));
             }
         }
         return s.top();
     }
 };