acm模式实现树的遍历

Posted 2022-10-15 王六六同学

今天的笔试题，acm模式写树，第一次遇到，关于树的输入输出格式做一个记录。

题目描述
分别按照二叉树先序，中序和后序打印所有的节点。

输入描述:
第一行输入两个整数 n 和 root，n 表示二叉树的总节点个数，root 表示二叉树的根节点。
以下 n 行每行三个整数 fa，lch，rch，表示 fa 的左儿子为 lch，右儿子为 rch。(如果 lch 为 0 则表示 fa 没有左儿子，rch同理)

输出描述:
输出三行，分别表示二叉树的先序，中序和后序。

递归：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * @author WanZi
 * @create 2022-10-11 16:33
 */
//二叉树的前序、中序、后序、遍历
public class Tree 

    public static StringBuilder pre = new StringBuilder();
    public static StringBuilder in = new StringBuilder();
    public static StringBuilder post = new StringBuilder();
    public static void main(String[] args) throws IOException 
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        reader.readLine();

        TreeNode root = creat(reader);

        preOrder(root);
        inOrder(root);
        postOrder(root);

        System.out.println();
        System.out.println("前序：" + pre.toString().trim());
        System.out.println("中序：" + in.toString().trim());
        System.out.println("后序：" + post.toString().trim());



    

    public static TreeNode creat(BufferedReader reader)
        String[] s = new String[0];
        try 
            s = reader.readLine().trim().split(" ");
            int val = Integer.parseInt(s[0]);
            int left = Integer.parseInt(s[1]);
            int right = Integer.parseInt(s[2]);

            TreeNode root = new TreeNode(val);
            if(left != 0)
                root.left = creat(reader);
            

            if(right != 0)
                root.right = creat(reader);
            
            return root;
         catch (IOException e) 
            return null;
        

    

    public static void preOrder(TreeNode root)
        if(root == null)
            return;

        
        pre.append(root.val + " ");
        preOrder(root.left);
        preOrder(root.right);
    


    public static void inOrder(TreeNode root)
        if(root == null)
            return;

        

        inOrder(root.left);
        in.append(root.val + " ");
        inOrder(root.right);
    

    public static void postOrder(TreeNode root)
        if(root == null)
            return;

        

        postOrder(root.left);
        postOrder(root.right);
        post.append(root.val + " ");
    


class TreeNode
    int val;
    TreeNode left;
    TreeNode right;

    public TreeNode(int val) 
        this.val = val;
    

非递归：

//前序
public static void preOrder(TreeNode root)
        if(root==null) return ;
        Stack<TreeNode> s = new Stack<>();
        TreeNode p = root;
        while(p!=null||!s.isEmpty())
            while(p!=null)
                pre.append(p.val+" ");
                s.push(p);
                p = p.left;
            
            p = s.pop();
            p = p.right;                        
        
    
public static void preOrder(TreeNode root)
	if(root==null) return ;
    Stack<TreeNode> s = new Stack<>();
    s.push(root);
    while(!s.isEmpty())
    	root = s.pop();
        pre.append(root.val+" ");
        if(root.right!=null)
 	       s.push(root.right);
        
        if(root.left!=null)
 	       s.push(root.left);
                               
   

//中序
public static void inOrder(TreeNode root)
    if(root==null) return ;
    Stack<TreeNode> s = new Stack<>();
    TreeNode p = root;
    while(p!=null||!s.isEmpty())
        while(p!=null)
            s.push(p);
            p = p.left;
        
        p = s.pop();
        in.append(p.val+" ");
        p = p.right;                        
    

//后序
public static void postOrder(TreeNode root)
   if(root==null) return ;
      Stack<TreeNode> s1 = new Stack<>();
      Stack<TreeNode> s2 = new Stack<>();
      s1.push(root);
      while(!s1.isEmpty())
          root = s1.pop();
          s2.push(root);
          if(root.left!=null)
              s1.push(root.left);
          
          if(root.right!=null)
              s1.push(root.right);
          
      
      while(!s2.isEmpty())
          post.append(s2.pop().val+" ");
      

public static void postOrder(TreeNode root)
      if(root==null) return ;
      Stack<TreeNode> s = new Stack<>();
      TreeNode c = null;
      s.push(root);
      while(!s.isEmpty())
          c = s.peek();
          if(c.left!=null&&c.left!=root&&c.right!=root)
              s.push(c.left);
          else if(c.right!=null&&c.right!=root)
              s.push(c.right);
          else
              c = s.pop();
              post.append(c.val+" ");
              root = c;