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：
1 import java.util.*; 2 3 public class Main { 4 static Scanner sc = new Scanner(System.in); 5 6 public static int lastvalue(String str){ //后缀表达式求值 7 Stack stack = new Stack (); 8 String[] data = str.split(" "); 9 10 for(int i = 0;i < data.length;i++){11 if(data[i].charAt(0) >= '0' && data[i].charAt(0) <= '9'){12 //将data[i]转换为int数据13 int m = Integer.parseInt(data[i]);14 stack.push(m);15 }else{16 int a = stack.pop();17 int b = stack.pop();18 //构造b ? a计算结果,，将结果入栈19 switch(data[i].charAt(0)){20 case '+':stack.push(b+a);break;21 case '-':stack.push(b-a);break;22 case '*':stack.push(b*a);break;23 case '/':stack.push(b/a);break;24 case '%':stack.push(b%a);break;25 }26 }27 // System.out.println("---list:"+stack);//测试输出栈内的情况28 }29 return stack.peek(); //程序运行结束后，栈内只剩一个数据，即答案30 }31 public static String turnvalue(String s){ //中缀变换后缀32 Stack stack = new Stack ();33 StringBuffer sb = new StringBuffer();34 int i = 0;35 while(i < s.length()){36 char c = s.charAt(i);37 switch(c){38 case '+':case '-': //存加减号39 while(!stack.isEmpty() && !stack.peek().equals("("))40 sb.append(stack.pop()+" ");41 stack.push(c + "");i++;42 break;43 case '*':case '/': //存乘除号44 while(!stack.isEmpty() && (stack.peek().equals("*") || stack.peek().equals("/")))45 sb.append(stack.pop()+" ");46 stack.push(c + "");i++;47 break;48 case '(':49 stack.push(c + "");i++;50 break;51 case ')':52 while(stack.peek() != null && !stack.peek().equals("("))53 sb.append(stack.pop() + " ");54 stack.pop();55 i++;break;56 default:57 if(c == ';'){ //可以存放一些结束符号58 i++;59 break;60 }61 while(i < s.length() && c >='0' && c <= '9'){ //保存数字62 sb.append(c);i++;63 if(i < s.length())64 c = s.charAt(i);65 }66 sb.append(" ");67 }68 }69 while(!stack.isEmpty())70 sb.append(stack.pop()+" ");71 // System.out.println(sb); //检查结果72 73 return sb.toString();74 }75 76 public static void main(String[] args) {77 while(sc.hasNext()){78 String s = sc.next();79 System.out.println(lastvalue(turnvalue(s)));80 }81 System.gc();sc.close();82 }83 }