[LeetCode 第4题] -- Evaluate Reverse Polish Notation

题目链接: Evaluate Reverse Polish Notation

题目意思: 给的一个表达式，求值

代码:

class Solution {
public:
     bool IsOperator(const string &str);
     int GetStackTop(stack<int> &stack);
     int evalRPN(vector<string> &tokens);
};

bool Solution::IsOperator(const string &str) {
     if (str == "+" || str == "-" ||
         str == "*" || str == "/") {
         return true;
     }
     return false;
}

int Solution::GetStackTop(stack<int> &stack) {
     if (stack.empty()) {
         throw "stack is empty";
         return -1;
     }
     int topValue = stack.top();
     stack.pop();
     return topValue;
}

int Solution::evalRPN(vector<string> &tokens) {
     stack <int> number;
     int tokensSize = tokens.size();
     for (int i = 0; i < tokensSize; ++i) {
         if (IsOperator(tokens[i])) {
             int rightNumber = GetStackTop(number);
             int leftNumber = GetStackTop(number);
             int result = 0;
             if (tokens[i] == "+") {
                result = leftNumber + rightNumber;
             }
             else if (tokens[i] == "-") {
                result = leftNumber - rightNumber;
             }
             else if (tokens[i] == "*") {
                result = leftNumber * rightNumber;
             }
             else {
                result = leftNumber / rightNumber;
             }
             number.push(result);
         }
         else {
             number.push(atoi(tokens[i].c_str()));
         }
      }
      return GetStackTop(number);
}