任何人都可以告诉我为什么在我的evaulateExpression中使用pop（）不起作用？

Only worried about getting the first test done in the Test Class since the root of the tree is an operator "+" and its are operands/"children" are 3 and 4. Since the root is "+" I want to pop the left child and right child and push the node to the stack. Trying to understand why I cannot use the pop() method from the Stack class.

只担心在测试类中完成第一次测试,因为树的根是一个运算符“+”,它的操作数/“子”是3和4.因为根是“+”我想要弹出左边子项和右项并将节点推送到堆栈。试图理解为什么我不能使用Stack类中的pop()方法。

Node Class

public class Node < E > {
 E data;
 Node < E > left;
 Node < E > right;

 public Node(E data) {
  this.data = data;
 }

 public Node(E data, Node < E > left, Node < E > right) {
  this.data = data;
  this.left = left;
  this.right = right;
 }

 public String toString() {
  return data.toString();
 }

}

ExpressionTree Class import java.util.Stack;

ExpressionTree类导入java.util.Stack;

public class ExpressionTree {
 Node < String > root;

 public void buildTree(String expression) {
  Stack < Node < String >> s = new Stack < Node < String >> ();

  String expArray[] = expression.split(" ");
  for (String st: expArray) {
   switch (st) {
    case "+":
    case "-":
    case "*":
    case "/":
     Node < String > right = s.pop();
     s.push((new Node < String > (st, s.pop(), right)));
     break;
    default:
     s.push(new Node < String > (st));


   }
  }

  root = s.pop();
 }

 public void printExpression() {
  printExpression(root);
  System.out.println();
 }

 private void printExpression(Node < String > n) {
  if (n != null) {
   printExpression(n.left);
   System.out.print(n);
   printExpression(n.right);
  }
 }

 public int evaluateExpression() {

  return evaluateExpression(root);
 }

 public int evaluateExpression(Node < String > n) {
  Stack < Node < String >> s = new Stack < Node < String >> ();
  n = root;
  if (n == null) {
   return 0;
  } else {
   if (n.data.equals("+")) {
    s.pop(n.left);
    s.pop(n.right);
    s.push(n);
    evaluateExpression(n);
   }
  }
  return 0;
 }

}

Test Class

public class ExpressionTreeTest {

 public static void main(String[] args) {
  ExpressionTree et = new ExpressionTree();
  et.buildTree("3 4 +"); //infix: 3 + 4
  et.printExpression();
  System.out.println(et.evaluateExpression());

  /*et.buildTree("3 4 2 * 1 5 - / +"); //infix: 3+4*2/(1-5)
    et.printExpression();
    System.out.println(et.evaluateExpression());

    et.buildTree("3 4 5 * 2 / +"); //infix: 3+4*5/2
    et.printExpression();
    System.out.println(et.evaluateExpression());

    et.buildTree("12 8 + 6 5 - * 3 2 - 2 3 + * /"); //infix: (12+8)*(6-
    5)/((3-2)*(2+3))
    et.printExpression();
    System.out.println(et.evaluateExpression());*/


 }

}

1 个解决方案

#1

As of the code

截至代码

s.pop(n.left);
s.pop(n.right);

As the Stack API shows, the pop function does NOT take any parameter.

正如Stack API所示,pop函数不接受任何参数。

It "[r]emoves the object at the top of this stack and returns that object as the value of this function."

它“[r]移动该堆栈顶部的对象,并将该对象作为此函数的值返回。”

You should use another class if you want to remove a specific object instead of whatever is on the top of the stack.

如果要删除特定对象而不是堆栈顶部的任何对象,则应使用另一个类。

#1