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

使用堆来做，遇到数字插入stack，遇到符号则pop两个元素进行运算，然后把结果push进stack。 注意先pop出来的是第二个数字。

 1 public class Solution {

 2     public int evalRPN(String[] tokens) {

 3         Stack<Integer> st = new Stack<Integer>();

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

 5             if(tokens[i].equals("+")){

 6                 int second = Integer.valueOf(st.pop());

 7                 int first = Integer.valueOf(st.pop());

 8                 st.push(first + second);

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

10                 int second = Integer.valueOf(st.pop());

11                 int first = Integer.valueOf(st.pop());

12                 st.push(first - second);               

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

14                 int second = Integer.valueOf(st.pop());

15                 int first = Integer.valueOf(st.pop());

16                 st.push(first * second);  

17             }else if(tokens[i].equals("/")){

18                 int second = Integer.valueOf(st.pop());

19                 int first = Integer.valueOf(st.pop());

20                 st.push((Integer)(first / second));  

21             }else{

22                 st.push(Integer.valueOf(tokens[i]));

23             }

24         }

25         return st.pop();

26     }

27 }

 第二遍：

 1 public class Solution {

 2     public int evalRPN(String[] tokens) {

 3         Stack<Integer> stack = new Stack<Integer>();

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

 5             if(tokens[i].equals("+")){

 6                 stack.push(stack.pop() + stack.pop());

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

 8                 int a = stack.pop();

 9                 stack.push(stack.pop() - a);

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

11                 stack.push(stack.pop() * stack.pop());

12             }else if(tokens[i].equals("/")){

13                 int a = stack.pop();

14                 stack.push((Integer)(stack.pop() / a));

15             }else

16                 stack.push(Integer.valueOf(tokens[i]));

17         }

18         return stack.pop();

19     }

20 }