150. Evaluate Reverse Polish Notation

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

Valid operators are

+

,

-

,

*

,

/

.
Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

思路:就是用栈来实现后缀式也就是逆波兰式的计算过程

代码如下(已通过leetcode)

public class Solution {

public int evalRPN(String[] tokens) {

Stack<String> stack=new Stack<String>();

for(String s:tokens) {

if(s.equals("+")||s.equals("-")||s.equals("*")||s.equals("/")) {

int dig2=Integer.parseInt(stack.pop());

int dig1=Integer.parseInt(stack.pop());

switch(s){

case "+":

stack.push(""+(dig1+dig2));

break;

case "-":

stack.push(""+(dig1-dig2));

break;

case "*":

stack.push(""+(dig1*dig2));

break;

case "/":

stack.push(""+(dig1/dig2));

break;

}

} else {

stack.push(s);

}

}

return Integer.parseInt(stack.pop());

}

}