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

December LeetCoding Challenge 17

Description

Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, and /. Each operand may be an integer or another expression.

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

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
// 22/22 cases passed (10 ms)
// Your runtime beats 91.72 % of cpp submissions
// Your memory usage beats 78.97 % of cpp submissions (12 MB)