实现一个二叉树的前序遍历、中序遍历和后序遍历方法。

package test3;public class Test_A27 {// 前序遍历（根-左-右）public void preOrderTraversal(TreeNode root){if(root==null){return;}System.out.println(root.val+"");preOrderTraversal(root.left);preOrderTraversal(root.right);}// 中序遍历（左-根-右）public void inOrderTraversal(TreeNode root){if(root==null){return;}preOrderTraversal(root.left);System.out.println(root.val+"");preOrderTraversal(root.right);}// 后序遍历（左-右-根）public void postOrderTraversal(TreeNode root){if(root==null){return;}preOrderTraversal(root.left);preOrderTraversal(root.right);System.out.println(root.val+"");}
public static void main(String[] args){TreeNode root=new TreeNode(11);TreeNode root2=new TreeNode(21);TreeNode root3=new TreeNode(31);TreeNode root4=new TreeNode(41);TreeNode root5=new TreeNode(51);TreeNode root6=new TreeNode(61);root.left=root2;root.right=root3;root2.left=root4;root2.right=root5;root3.right=root6;Test_A27 traversal=new Test_A27();System.out.println("前序遍历结果：");traversal.preOrderTraversal(root);System.out.println("中序遍历结果：");traversal.inOrderTraversal(root);System.out.println("后序遍历结果：");traversal.postOrderTraversal(root);
}}class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode(int val){this.val=val;}}