LeetCode in TypeScript
150. Evaluate Reverse Polish Notation
Medium
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 <= 104tokens[i]is either an operator:"+","-","*", or"/", or an integer in the range[-200, 200].
Solution
function evalRPN(tokens: string[]): number {
const numberStack: number[] = []
const isOperator = (val: string): boolean => val === '+' || val === '-' || val === '*' || val === '/'
for (let token of tokens) {
if (isOperator(token)) {
const right = numberStack.pop() as number
const left = numberStack.pop() as number
if (token === '+') {
numberStack.push(left + right)
} else if (token === '-') {
numberStack.push(left - right)
} else if (token === '*') {
numberStack.push(left * right)
} else if (token === '/') {
const result = left / right
numberStack.push(result < 0 ? Math.ceil(result) : Math.floor(result))
}
} else numberStack.push(+token)
}
return numberStack.pop() || 0
}
export { evalRPN }