LeetCode2EvaluateReversePolishNotation

Evaluate the value of arithmetic expression in Reverse Polish Notation. Valid operator are ,-,*,/. Each operand may be an integer or another expression. Some examples: [2, 1, , 3, *] - ((21)*3) - 9 [4, 13, 5, /, ] - (4 (13 / 5)) - 6 分析：

Evaluate the value of arithmetic expression in Reverse Polish Notation.

Valid operator 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

分析：后缀表达式操作。

栈的应用，如果碰见数字，则压栈，碰见运算符则弹出两个元素，对两个元素进行数学运算后结果压栈。

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack st = new Stack();
        for(String token : tokens){
            if(token.matches("-?[0-9]+")){
                st.push(Integer.parseInt(token));
            }else{
                int num2 = st.pop();
                int num1 = st.pop();
                if(token.equals("+")){
                    st.push(num1+num2);
                }else if(token.equals("-")){
                    st.push(num1-num2);
                }else if(token.equals("*")){
                    st.push(num1*num2);
                }else if(token.equals("/")){
                    st.push(num1/num2);
                }
            } 
        }
        return st.pop();
    }
}