package stack_queue;

import org.junit.Test;

import java.util.Stack;

/**

* @Author: wei1

* @Date: Create in 2018/12/2 16:55

* @Description: 1.这种逆波兰表达式，很明显，用栈求解。

* Evaluate the value of an arithmetic expression in Reverse Polish Notation.

* <p>

* Valid operators are+,-,*,/. Each operand may be an integer or another expression.

* <p>

* Some examples:

* <p>

* ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

* ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

*/

public class EvalRPN {

public int evalRPN(String[] tokens) {

Stack<Integer> stack = new Stack();

for (int t = 0; t < tokens.length; t++) {

try {

int i = Integer.parseInt(tokens[t]);

stack.push(i);

} catch (Exception e) {

int b = stack.pop();

int a = stack.pop();

if (tokens[t].equals("+")) {

stack.add(a + b);

} else if (tokens[t].equals("-")) {

stack.add(a - b);

} else if (tokens[t].equals("*")) {

stack.add(a * b);

} else {

stack.add(a / b);

}

}

}

return stack.peek();

}

@Test

public void test() {

String[] strs = {"2", "1", "+", "3", "*"};

System.out.println(evalRPN(strs));

}

}