LeetCode | Evaluate Reverse Polish Notation

The algorithm for evaluating any postfix expression is fairly straightforward:

While there are input tokens left

• • Read the next token from input.
  • If the token is a value
    • Push it onto the stack.
  • Otherwise, the token is an operator (operator here includes both operators and functions).
    • It is known a priori that the operator takes n arguments.
    • If there are fewer than n values on the stack
      • (Error) The user has not input sufficient values in the expression.
    • Else, Pop the top n values from the stack.
    • Evaluate the operator, with the values as arguments.
    • Push the returned results, if any, back onto the stack.
• If there is only one value in the stack
  • That value is the result of the calculation.
• Otherwise, there are more values in the stack
  • (Error) The user input has too many values.

 1 class Solution {

 2 public:

 3     int evalRPN(vector<string> &tokens) {

 4         if (tokens.empty()) return 0;

 5         

 6         stack<int> st;

 7         

 8         for (int i = 0; i < tokens.size(); ++i) {

 9             if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {

10                 if (st.empty()) return 0;

11                 int n1 = st.top();

12                 st.pop();

13                 if (st.empty()) return 0;

14                 int n2 = st.top();

15                 st.pop();

16                 if (tokens[i] == "+") st.push(n1 + n2);

17                 else if (tokens[i] == "-") st.push(n2 - n1);

18                 else if (tokens[i] == "*") st.push(n2 * n1);

19                 else if (n1 == 0) return 0;

20                 else st.push(n2 / n1);

21             } else {

22                 st.push(atoi(tokens[i].c_str()));

23             }

24         }

25         

26         return st.top();

27     }

28 };