HackMD
150.Evaluate Reverse Polish Notation
tags: Medium,Array,Math,Stack
150. Evaluate Reverse Polish Notation
題目描述
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.
範例
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].
解答
C#
public class Solution {
public int EvalRPN(string[] tokens) {
Stack<int> operands = new();
var operators = new string[] { "+", "-", "*", "/" };
foreach (string token in tokens)
{
if (operators.Contains(token))
{
int operand2 = operands.Pop();
int operand1 = operands.Pop();
operands.Push(Calc(operand1, operand2, token));
}
else
{
operands.Push(int.Parse(token));
}
}
return operands.Pop();
static int Calc(int operand1, int operand2, string oper) => oper switch
{
"+" => operand1 + operand2,
"-" => operand1 - operand2,
"*" => operand1 * operand2,
"/" => operand1 / operand2,
_ => throw new ArgumentException(null, nameof(oper)),
};
}
}
Jim Dec 17, 2022
Python
class Solution:
def evalRPN(self, tokens: List[str]) -> int:
stk = []
for token in tokens:
if token in "+-*/":
num2 = stk.pop()
num1 = stk.pop()
if token == '+':
stk.append(num1 + num2)
elif token == '-':
stk.append(num1 - num2)
elif token == '*':
stk.append(num1 * num2)
elif token == '/':
stk.append(int(num1 / num2))
else:
stk.append(int(token))
return stk.pop()
