实现二叉树先序，中序和后序遍历

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

/**
 * @描述：遍历二叉树
 * @思路：
 * @复杂度：
 * @链接： https://www.nowcoder.com/practice/566f7f9d68c24691aa5abd8abefa798c
 */
 class TraversalBinaryTree {


    public static void preOrderTraversal(TreeNode root) {
        if(root==null) {
            return;
        }
        System.out.print(root.value + " ");
        if (root.left != null) {
            preOrderTraversal(root.left);
        }
        if (root.right != null) {
            preOrderTraversal(root.right);
        }
    }


    public static void inOrderTraversal(TreeNode root) {
        if (root.left != null) {
            inOrderTraversal(root.left);
        }
        System.out.print(root.value + " ");
        if (root.right != null) {
            inOrderTraversal(root.right);
        }
    }

    public static void postOrderTraversal(TreeNode root) {
        if(root==null) {
            return;
        }
        if (root.left != null) {
            postOrderTraversal(root.left);
        }
        if (root.right != null) {
            postOrderTraversal(root.right);
        }
        System.out.print(root.value + " ");
    }




}


public class Main {

    public static void main(String[] args) throws Exception {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String[] firstRow = in.readLine().split(" ");
        int row = Integer.parseInt(firstRow[0]);
        String[] nodeValueArr =  new String[row * 3];

        TreeNode root = TreeUtils.makeBinaryTreeByInput(in);
//        System.out.println("先序遍历：");
        TraversalBinaryTree.preOrderTraversal(root);
//        System.out.println("\n" + "中序遍历：");
        System.out.println();
        TraversalBinaryTree.inOrderTraversal(root);
//        System.out.println("\n" + "后序遍历：");
        System.out.println();
        TraversalBinaryTree.postOrderTraversal(root);
    }

}





class TreeUtils {

    private static TreeNode root;


    /**
     * 采用递归的方式创建一颗二叉树
     *    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)
     * 构造后是二叉树的二叉链表表示法
     */
    //创建一个树的结点
    public static TreeNode<String> makeBinaryTreeByInput(BufferedReader in) throws Exception{
        String[] nodeVals = in.readLine().split(" ");
        //数组中第一个数是根节点
        TreeNode<String> node = new TreeNode<>(nodeVals[0]);
        //通过递归确定了层数
        if (!"0".equals(nodeVals[1])) {
            node.left = makeBinaryTreeByInput(in);
        }
        if (!"0".equals(nodeVals[2])) {
            node.right = makeBinaryTreeByInput(in);
        }
        return node;
    }


}



 class TreeNode<T> {
   public T value;
   public TreeNode<T> left;
   public TreeNode<T> right;

    public TreeNode(T value) {
        this.value = value;
    }

    public TreeNode( ) {
    }

    public TreeNode(TreeNode left, T value, TreeNode right) {
        this.value = value;
        this.left = left;
        this.right = right;
    }
}