[LeetCode]Evaluate Reverse Polish Notation

题目描述：

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

解题方案：

该题借助一个栈，遍历tokens时，如果当前不为"+","-","*","/"，则入栈，否则将栈顶的两个元素出栈后，运算后结果入栈。下面是该题代码：

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> result;
        int a, b;
        for (vector<string>::iterator ix = tokens.begin(); ix != tokens.end(); ++ix) {
            if (*ix == "+" || *ix == "-" || *ix == "*" || *ix == "/") {
                a = result.top();
                result.pop();
                b = result.top();
                result.pop();
                if (*ix == "+") {
                    result.push(b + a);
                }
                if (*ix == "-") {
                    result.push(b - a);
                }
                if (*ix == "*") {
                    result.push(b * a);
                }
                if (*ix == "/") {
                    result.push(b / a);
                }
            } else {
                result.push(atoi((*ix).c_str()));
            }
        }
        return result.top();
    }
};