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 }