[Leetcode] 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

 

Solution:

public class Solution {
    public int evalRPN(String[] tokens) {
        if(tokens==null)
            return 0;
        Stack<Integer> s=new Stack<Integer>();
        for(int i=0;i<tokens.length;++i){
            if(isNumber(tokens[i])){
                s.add(Integer.parseInt(tokens[i]));
            }else{
                int param_1=s.pop();
                int param_2=s.pop();
                s.add(calc(param_1,param_2,tokens[i]));
            }
        }
        return s.pop();
    }
    private int calc(int param_1, int param_2, String op) {
        // TODO Auto-generated method stub
        switch (op.charAt(0)) {
        case '+':
            return param_1+param_2;
        case '-':
            return param_2-param_1;
        case '*':
            return param_1*param_2;
        case '/':
            return param_2/param_1;
        default:
            return 0;
        }
    }
    private boolean isNumber(String s) {
        // TODO Auto-generated method stub
        try {
            int i_s=Integer.parseInt(s);
        } catch (Exception e) {
            // TODO: handle exception
            return false;
        }
        return true;
    }
}

需要注意：

判断是不是数字：

private boolean isNumber(String s) {
        // TODO Auto-generated method stub
        try {
            int i_s=Integer.parseInt(s);
        } catch (Exception e) {
            // TODO: handle exception
            return false;
        }
        return true;
}