算法：94. 二叉树的中序遍历--扩展前中后层序遍历

中序遍历 

给定一个二叉树的根节点 root ，返回 它的 中序 遍历 。

中遍历结果为：H D I B E J A F K C G
 

示例 1：

输入：root = [1,null,2,3]
输出：[1,3,2]

示例 2：

输入：root = []
输出：[]

示例 3：

输入：root = [1]
输出：[1]

提示：

• 树中节点数目在范围 [0, 100] 内
• -100 <= Node.val <= 100

方法一：递归

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        inorder(root, res);
        return res;
    }

    public void inorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        inorder(root.left, res);
        res.add(root.val);
        inorder(root.right, res);
    }
}

方法二：迭代

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        Deque<TreeNode> stk = new LinkedList<TreeNode>();
        while (root != null || !stk.isEmpty()) {
            while (root != null) {
                stk.push(root);
                root = root.left;
            }
            root = stk.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }
}

前序遍历  

先序遍历结果为：A B D H I E J C F K G

import java.util.Stack;

// 定义二叉树节点
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    
    TreeNode(int val) {
        this.val = val;
    }
}

public class PreorderTraversal {
    public static void preorderTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode current = stack.pop();
            System.out.print(current.val + " ");
            
            // 先将右子树入栈，再将左子树入栈，这样出栈的顺序就是先左后右
            if (current.right != null) {
                stack.push(current.right);
            }
            if (current.left != null) {
                stack.push(current.left);
            }
        }
    }
    
    public static void main(String[] args) {
        // 创建二叉树
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        
        // 非递归前序遍历
        System.out.println("非递归前序遍历结果：");
        preorderTraversal(root);
    }
}

后序遍历 

       
后序遍历结果：H I D J E B K F G C A

import java.util.Stack;

// 定义二叉树节点
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    
    TreeNode(int val) {
        this.val = val;
    }
}

public class PostorderTraversal {
    public static void postorderTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        Stack<TreeNode> output = new Stack<>(); // 用于保存后序遍历结果
        
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode current = stack.pop();
            output.push(current);
            
            if (current.left != null) {
                stack.push(current.left);
            }
            if (current.right != null) {
                stack.push(current.right);
            }
        }
        
        // 输出后序遍历结果
        while (!output.isEmpty()) {
            TreeNode node = output.pop();
            System.out.print(node.val + " ");
        }
    }
    
    public static void main(String[] args) {
        // 创建二叉树
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        
        // 非递归后序遍历
        System.out.println("非递归后序遍历结果：");
        postorderTraversal(root);

//非递归后序遍历结果：
//4 5 2 3 1
    }
}

层序遍历   

层次遍历结果：A B C D E F G H I J K

自己的总结及扩展： 算法：二叉树的层序遍历和每层最大值-优快云博客

https://blog.youkuaiyun.com/modi000/article/details/127301630