LeetCode-in-Java
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
import java.util.Stack;
@SuppressWarnings("java:S1149")
public class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> st = new Stack<>();
for (String token : tokens) {
if (!Character.isDigit(token.charAt(token.length() - 1))) {
st.push(eval(st.pop(), st.pop(), token));
} else {
st.push(Integer.parseInt(token));
}
}
return st.pop();
}
private int eval(int second, int first, String operator) {
switch (operator) {
case "+":
return first + second;
case "-":
return first - second;
case "*":
return first * second;
default:
return first / second;
}
}
}