表达式求值

这段代码用到的是栈的方法，先将中缀表达式改为后缀表达式，再对后缀表达式求值。

不多说，看代码：

import java.util.*;

public class Main {
    static Scanner sc = new Scanner(System.in);
    
    public static int lastvalue(String str){    //后缀表达式求值
        Stack<Integer> stack = new Stack<Integer>();
        String[] data = str.split(" ");
         
        for(int i = 0;i < data.length;i++){
            if(data[i].charAt(0) >= '0' && data[i].charAt(0) <= '9'){
                //将data[i]转换为int数据
                int m =  Integer.parseInt(data[i]);
                stack.push(m);
            }else{
                int a = stack.pop();
                int b = stack.pop();
                //构造b ? a计算结果,，将结果入栈
                switch(data[i].charAt(0)){
                    case '+':stack.push(b+a);break;
                    case '-':stack.push(b-a);break;
                    case '*':stack.push(b*a);break;
                    case '/':stack.push(b/a);break;
                    case '%':stack.push(b%a);break;
                }
            }
//          System.out.println("---list:"+stack);//测试输出栈内的情况
        }
        return stack.peek();    //程序运行结束后，栈内只剩一个数据，即答案
    }
    public static String turnvalue(String s){   //中缀变换后缀
        Stack<String> stack = new Stack<String>();
        StringBuffer sb = new StringBuffer();
        int i = 0;
        while(i < s.length()){
            char c = s.charAt(i);
            switch(c){
                case '+':case '-':    //存加减号
                    while(!stack.isEmpty() && !stack.peek().equals("("))
                        sb.append(stack.pop()+" ");
                    stack.push(c + "");i++;
                    break;
                case '*':case '/':    //存乘除号
                    while(!stack.isEmpty() && (stack.peek().equals("*") || stack.peek().equals("/")))
                        sb.append(stack.pop()+" ");
                    stack.push(c + "");i++;
                    break;
                case '(':
                    stack.push(c + "");i++;
                    break;
                case ')':
                    while(stack.peek() != null && !stack.peek().equals("("))
                        sb.append(stack.pop() + " ");
                    stack.pop();
                    i++;break;
                default:
                    if(c == ';'){    //可以存放一些结束符号
                        i++;
                        break;
                    }
                    while(i < s.length() && c >='0' && c <= '9'){    //保存数字
                        sb.append(c);i++;
                        if(i < s.length())
                            c = s.charAt(i);
                    }
                    sb.append(" ");
            }
        }
        while(!stack.isEmpty())
            sb.append(stack.pop()+" ");
//      System.out.println(sb);        //检查结果
         
        return sb.toString();
    }
    
    public static void main(String[] args) {
        while(sc.hasNext()){
            String s = sc.next();
            System.out.println(lastvalue(turnvalue(s)));
        }
        System.gc();sc.close();
    }
}