程序员代码面试指南刷题--第三章.实现二叉树先序，中序和后序遍历

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。

示例1
输入

3 1
1 2 3
2 0 0
3 0 0

输出

1 2 3
2 1 3
2 3 1

解法一：递归

import java.io.*;
import java.util.*;
public class Main{
    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 Exception{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        br.readLine();
        TreeNode root = createTree(br);
        //先序
        preOrder(root);
        //中序
        inOrder(root);
        //后续
        postOrder(root);
        System.out.println(pre.toString().trim());
        System.out.println(in.toString().trim());
        System.out.println(post.toString().trim());
    }
    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+" ");
    }
    public static TreeNode createTree(BufferedReader br){
        try{
            String[] ss = br.readLine().trim().split(" ");
            int data = Integer.parseInt(ss[0]);
            int left = Integer.parseInt(ss[1]);
            int right = Integer.parseInt(ss[2]);
            TreeNode root = new TreeNode(data);
            if(left!=0){
                root.left = createTree(br);
            }
            if(right!=0){
                root.right = createTree(br);
            }
            return root;
        }catch(Exception e){
            return null;
        }
    }
}
class TreeNode{
    int val;
    TreeNode left;
    TreeNode right;
    public TreeNode(int val){
        this.val = val;
    }
}

解法二：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;
          }
      }
}