Evaluate Reverse Polish Notation LeetCode Solution | Easy Approach

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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].

Evaluate Reverse Polish Notation Solutions

Time: O(n)
Space: O(n)

C++

class Solution {
 public:
  int evalRPN(vector<string>& tokens) {
    stack<int> stack;
    const unordered_map<string, function<int(int, int)>> op{
        {"+", plus<int>()},
        {"-", minus<int>()},
        {"*", multiplies<int>()},
        {"/", divides<int>()}};

    for (const string& token : tokens)
      if (op.count(token)) {
        const int b = stack.top();
        stack.pop();
        const int a = stack.top();
        stack.pop();
        stack.push(op.at(token)(a, b));
      } else {
        stack.push(stoi(token));
      }

    return stack.top();
  }
};

Java

 class Solution {
  public int evalRPN(String[] tokens) {
    Deque<Integer> stack = new ArrayDeque<>();

    for (final String token : tokens)
      switch (token) {
        case "+":
          stack.push(stack.pop() + stack.pop());
          break;
        case "-":
          stack.push(-stack.pop() + stack.pop());
          break;
        case "*":
          stack.push(stack.pop() * stack.pop());
          break;
        case "/":
          final int b = stack.pop();
          final int a = stack.pop();
          stack.push(a / b);
          break;
        default:
          stack.push(Integer.parseInt(token));
      }

    return stack.peek();
  }
}

Python


class Solution:
  def evalRPN(self, tokens: List[str]) -> int:
    stack = []
    operators = {
        '+': lambda a, b: a + b,
        '-': lambda a, b: a - b,
        '*': lambda a, b: a * b,
        '/': lambda a, b: int(a / b),
    }

    for token in tokens:
      if token in operators:
        b = stack.pop()
        a = stack.pop()
        stack.append(operators[token](a, b))
      else:
        stack.append(int(token))

    return stack[0]

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