LeetCode-in-Go
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
func evalRPN(tokens []string) int {
stack := make([]int, 0)
for _, token := range tokens {
if len(token) == 1 && !isDigit(token[0]) {
second := stack[len(stack)-1]
first := stack[len(stack)-2]
stack = stack[:len(stack)-2]
stack = append(stack, eval(first, second, token))
} else {
num := 0
sign := 1
i := 0
if token[0] == '-' {
sign = -1
i = 1
}
for ; i < len(token); i++ {
num = num*10 + int(token[i]-'0')
}
stack = append(stack, num*sign)
}
}
return stack[0]
}
func eval(first, second int, operator string) int {
switch operator {
case "+":
return first + second
case "-":
return first - second
case "*":
return first * second
default:
return first / second
}
}
func isDigit(c byte) bool {
return c >= '0' && c <= '9'
}