150逆波兰式

150.Evaluate Reverse Polish Notation

Medium

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

Solution

import java.util.Stack;

class Solution {
   public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0; i < tokens.length; i++){
            if(tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")){
                int n1 = stack.pop() ;
                int n2 = stack.pop();
                if(tokens[i].equals("+"))
                {
                    stack.push(n1 + n2);
                }
                if(tokens[i].equals("-"))
                {
                    stack.push(n2 - n1);
                }
                if(tokens[i].equals("*"))
                {
                    stack.push(n1 * n2);
                }
                if(tokens[i].equals("/"))
                {
                    stack.push(n2 / n1);
                }

            }else{
                stack.push(Integer.parseInt(tokens[i]));
            }
        }
        return stack.pop();
    }

}

kens[i]));
}
}
return stack.pop();
}

}