力扣算法题—150. 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.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

Solution:
　　使用栈即可

 class Solution {
 public:
     int evalRPN(vector<string> &tokens) {
         if (tokens.size() == )return ;
         stack<int>s;
         for (auto a : tokens)
         {
             if (a == "+" || a == "-" || a == "*" || a == "/")
             {
                 int num2 = s.top();
                 s.pop();
                 int num1 = s.top();
                 s.pop();
                 int res = ;
                 if (a == "+")
                     res = num1 + num2;
                 else if (a == "-")
                     res = num1 - num2;
                 else if (a == "*")
                     res = num1 * num2;
                 else
                     res = num1 / num2;
                 s.push(res);
             }
             else
                 s.push(atoi(a.c_str()));
         }
         return s.top();
     }
 };