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() == 1 && !isdigit((*cIter)[0])){
                char op = (*cIter)[0];
                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();
    }
};