Today I have done leetcode's January LeetCoding Challenge with cpp.

January LeetCoding Challenge 30

Description

Evaluate Reverse Polish Notation

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

Example 1:

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

Example 2:

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

Example 3:

Input: tokens = ["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

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Solution

bool isOp(string &token) {
  return token.length() == 1 && !isdigit(token[0]);
}

int apply(string &op, long long op1, long long op2) {
  switch (op[0]) {
    case '+':
      return op1 + op2;
    case '-':
      return op1 - op2;
    case '*':
      return op1 * op2;
    case '/':
      return op1 / op2;
  }
  return -1;
}

class Solution {
public:
  int evalRPN(vector<string>& tokens) {
    vector<long long> st;
    for(auto &token : tokens) {
      if(isOp(token)) {
        int op2 = st.back();
        st.pop_back();
        int op1 = st.back();
        st.pop_back();
        st.push_back(apply(token, op1, op2));
      } else {
        st.push_back(stoi(token));
      }
    }
    return st.back();
  }
};

// Accepted
// 21/21 cases passed (3 ms)
// Your runtime beats 98.68 % of cpp submissions
// Your memory usage beats 10.51 % of cpp submissions (15.4 MB)