二叉树：
种类：满二叉树、完全二叉树、二叉搜索树、平衡二叉搜索树
存储方式：链式存储、线式存储（顺序存储）
二叉数遍历：深度优先搜索（前序、中序、后序）：使用递归实现（实际用栈来实现）、迭代法（非递归的方式、栈），广度优先搜索（层序遍历）：用队列
递归三步走写法：1、确定递归函数的参数和返回值。2、确定终止条件。3、确定单层递归的逻辑。
144、二叉树的前序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    // 方法一：递归法
    // public List<Integer> preorderTraversal(TreeNode root) {
    //     List<Integer> result = new ArrayList<>();
    //     preorder(root, result);
    //     return result;
    // }
    // public void preorder(TreeNode root, List<Integer> result){
    //     if(root == null){
    //         return;
    //     }
    //     result.add(root.val);
    //     preorder(root.left, result);
    //     preorder(root.right, result);
    // }
    // 方法二：非递归的方法
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            if(cur != null){
                result.add(cur.val);
                stack.push(cur.right);
                stack.push(cur.left);
            }else{
                continue;
            }
        }
        return result;
    }
// 方法三：统一风格的非递归
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root != null){
            stack.push(root);
        }
        while(!stack.isEmpty()){
            if(stack.peek() != null){
                TreeNode cur = stack.pop();
                if(cur.right != null){
                    stack.push(cur.right);
                }
                if(cur.left != null){
                    stack.push(cur.left);
                }
                stack.push(cur);
                stack.push(null);
            }else{
                stack.pop();
                TreeNode cur = stack.pop();
                result.add(cur.val);
            }
        }
        return result;
    }
}

145、二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    // 方法一：递归法
    // public List<Integer> postorderTraversal(TreeNode root) {
    //     List<Integer> result = new ArrayList<>();
    //     postOrder(root, result);
    //     return result;
    // }
    // public void postOrder(TreeNode root, List<Integer> result){
    //     if(root == null){
    //         return;
    //     }
    //     postOrder(root.left, result);
    //     postOrder(root.right, result);
    //     result.add(root.val);
    // }
    // 方法二：非递归
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if(root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            result.add(cur.val);
            if(cur.left != null){
                stack.push(cur.left);
            }
            if(cur.right != null){
                stack.push(cur.right);
            }
        }
        Collections.reverse(result);
        return result;
    }
// 方法三：统一风格的非递归
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root != null){
            stack.push(root);
        }
        while(!stack.isEmpty()){
            if(stack.peek() != null){
                TreeNode cur = stack.pop();
                stack.push(cur);
                sta