1.先定义节点

/*定义一个树节点*/
public class TreeNode {int val;         //存储值TreeNode left;   //左子树TreeNode right;  //右子树//无参构造方法TreeNode (){}//有参构造方法TreeNode(int val){this.val=val;}TreeNode(int val,TreeNode left,TreeNode right){this.val=val;this.left=left;this.right=right;}
}

2.手动创建一个二叉树并遍历。

此处同时使用了前序遍历、中序遍历、后序遍历，实际只使用一种方法即可。

public class BinaryTreeDemo {public static void main(String[] args) {//手动创建一个树TreeNode root=createTree();//1.前序遍历List<Integer> result=preorderTraversal(root);//2.中序遍历List<Integer> result2=inorderTraversal(root);//3.后序遍历List<Integer> result3=postorderTraversal(root);System.out.println(result);System.out.println(result2);System.out.println(result3);}//手动创建二叉树public static TreeNode createTree(){TreeNode root=new TreeNode(1);  //创建根节点TreeNode treeNode2=new TreeNode(2);TreeNode treeNode3=new TreeNode(3);TreeNode treeNode4=new TreeNode(4);TreeNode treeNode5=new TreeNode(5);TreeNode treeNode6=new TreeNode(6);TreeNode treeNode7=new TreeNode(7);root.left = treeNode2;root.right = treeNode3;treeNode2.left = treeNode4;treeNode2.right = treeNode5;treeNode3.left = treeNode6;treeNode3.right = treeNode7;return root;}//前序遍历public static List<Integer> preorderTraversal(TreeNode root) {List<Integer> result=new ArrayList<>();preorder(root,result);return result;}public static void preorder(TreeNode root,List<Integer> result){if(root==null) return;    //如果当前节点为空，结束递归result.add(root.val);    //不为空，则将当前节点的值，加入链表中preorder(root.left,result);preorder(root.right,result);}//中序遍历public static List<Integer> inorderTraversal(TreeNode root) {List<Integer> result=new ArrayList<>();inorder(root,result);return result;}public static void inorder(TreeNode root,List<Integer> result){if(root==null) return;inorder(root.left,result);result.add(root.val);inorder(root.right,result);}//后序遍历public static List<Integer> postorderTraversal(TreeNode root) {List<Integer> result=new ArrayList<>();postorder(root,result);return result;}public static void postorder(TreeNode root,List<Integer> result){if(root==null) return;postorder(root.left,result);postorder(root.right,result);result.add(root.val);}
}