Reverse Polish Notation - A Stack Application

Medium L.C. problem: https://leetcode.com/problems/evaluate-reverse-polish-notation/

Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +-*/. Each operand may be an integer or another expression.
Note:
  • Division between two integers should truncate toward zero.
  • The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:
Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: ["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

This is a classic application of Stack. Linear time, linear space. Use of Long just to avoid overflows. Code is below, cheers, ACC.

public class Solution
{
    public int EvalRPN(string[] tokens)
    {
        Stack stack = new Stack();

        foreach (string token in tokens)
        {
            string v1 = "";
            string v2 = "";
            long val = 0;
            switch (token)
            {
                case "+":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) + Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "-":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) - Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "*":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) * Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "/":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) / Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                default:
                    stack.Push(token);
                    break;
            }
        }

        return Int32.Parse(stack.Pop());
    }
}

Comments

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    ReplyDelete
  2. sweet, although most stack manipulations can be reused:

    from operator import add, sub, mul, truediv

    class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
    stack = []
    ops = {"+": add, "-": sub, "*": mul, "/": truediv}
    for token in tokens:
    if token in ops:
    y, x = stack.pop(), stack.pop()
    stack.append(int(ops[token](x, y)))
    else:
    stack.append(int(token))

    return stack[0]

    ReplyDelete
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