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: 9Explanation: ((2 + 1) * 3) = 9
Question
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: 9Explanation: ((2 + 1) * 3) = 9
Solution
To solve this problem, we can use a stack data structure. Here are the steps:
- Initialize an empty stack.
- Iterate over the tokens array from left to right.
- If the current token is a number, push it into the stack.
- If the current token is an operator (i.e., '+', '-', '*', '/'), pop the top two elements from the stack, perform the operation, and push the result back into the stack.
- After going through all the tokens, the stack should contain only one element, which is the result of the expression.
Here is a Python solution:
def evalRPN(tokens):
stack = []
for token in tokens:
if token in ["+", "-", "*", "/"]:
num2 = stack.pop()
num1 = stack.pop()
if token == "+":
stack.append(num1 + num2)
elif token == "-":
stack.append(num1 - num2)
elif token == "*":
stack.append(num1 * num2)
else:
stack.append(int(num1 / num2)) # note that we use int division to truncate towards zero
else:
stack.append(int(token))
return stack.pop()
For example, if we call evalRPN(["2","1","+","3","*"]), it will return 9.
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