题目
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Note:
- Division between two integers should truncate toward zero.
- The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:
Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: ["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
代码
class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> numberStack = new Stack<>();
for (String token : tokens) {
if (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/")) {
Integer right = numberStack.pop();
Integer left = numberStack.pop();
switch (token) {
case "+": {
numberStack.push(left + right);
}
break;
case "-": {
numberStack.push(left - right);
}
break;
case "*": {
numberStack.push(left * right);
}
break;
case "/": {
numberStack.push(left / right);
}
break;
default: {
numberStack.push(Integer.parseInt(token));
}
}
} else {
numberStack.push(Integer.parseInt(token));
}
}
return numberStack.pop();
}
}
