剑指 Offer II 036. 后缀表达式

题目：根据 逆波兰表示法，求该后缀表达式的计算结果。
有效的算符包括 +、-、*、/ 。每个运算对象可以是整数，也可以是另一个逆波兰表达式。

从左到右扫描数组，遇到操作数则入栈，遇到操作符则取出栈顶元素和第二个元素进行运算，运算完的的结果重新入栈直到遇到下一个操作符。

public static int evalRPN(String[] tokens) {
        Stack stack = new Stack<>();
        for (int i = 0; i < tokens.length; ++i){
            if (tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "*" && tokens[i] != "/"){
                int sum = Integer.valueOf(tokens[i]);
            	stack.push(sum);
            }else{
                int a = stack.pop();
                int b = stack.pop();
                if (tokens[i] == "+"){
                    int c = add(a, b);
                    stack.push(c);
                }else if (tokens[i] == "-"){
                    int c = jian(a, b);
                    stack.push(c);
                }else if (tokens[i] == "*"){
                    int c = cheng(a, b);
                    stack.push(c);
                }else if (tokens[i] == "/"){
                    int c = chu(a, b);
                    stack.push(c);
                }
            }
        }
        return stack.pop();
    }
    public static int add(int a, int b){
        return a + b;
    }
    public static int jian(int a, int b){
        return a - b;
    }
    public static int cheng(int a, int b){
        return a * b;
    }
    public static int chu(int a, int b){
        return a / b;
    }

优化后的：

public int evalRPN(String[] tokens) {
        Stack stack = new Stack<>();
        for (String c : tokens) {
            switch (c) {
                case "+":
                case "-":
                case "*":
                case "/":
                    int num2 = stack.pop();
                    int num1 = stack.pop();
                    stack.push(calc(num1, num2, c));
                    break;
                default:
                    stack.push(Integer.parseInt(c));
            }
        }
        return stack.pop();
    }

    private int calc(int a, int b, String op) {
        switch (op) {
            case "+":
                return a + b;
            case "-":
                return a - b;
            case "*":
                return a * b;
            default:
                return a / b;
        }
    }