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。

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> st = new Stack<Integer>();
        for(int i = 0; i < tokens.length; i ++){
            if(tokens[i].equals("+")){
                int second = Integer.valueOf(st.pop());
                int first = Integer.valueOf(st.pop());
                st.push(first + second);
            }else if(tokens[i].equals("-")){
                int second = Integer.valueOf(st.pop());
                int first = Integer.valueOf(st.pop());
                st.push(first - second);               
            }else if(tokens[i].equals("*")){
                int second = Integer.valueOf(st.pop());
                int first = Integer.valueOf(st.pop());
                st.push(first * second);  
            }else if(tokens[i].equals("/")){
                int second = Integer.valueOf(st.pop());
                int first = Integer.valueOf(st.pop());
                st.push((Integer)(first / second));  
            }else{
                st.push(Integer.valueOf(tokens[i]));
            }
        }
        return st.pop();
    }
}

 第二遍：

public 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("+")){
                stack.push(stack.pop() + stack.pop());
            }else if(tokens[i].equals("-")){
                int a = stack.pop();
                stack.push(stack.pop() - a);
            }else if(tokens[i].equals("*")){
                stack.push(stack.pop() * stack.pop());
            }else if(tokens[i].equals("/")){
                int a = stack.pop();
                stack.push((Integer)(stack.pop() / a));
            }else
                stack.push(Integer.valueOf(tokens[i]));
        }
        return stack.pop();
    }
}