二叉树迭代遍历JAVA

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;

//二叉树迭代遍历
public class TraversalTreeNode {

    public static void main(String[] args) {

    }

    //二叉树前序遍历
    public List preorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Deque stack = new ArrayDeque<>();
        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                res.add(node.val);
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            node = node.right;
        }
        return res;
    }

    //二叉树中序遍历
    public List inorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        TreeNode node = root;
        Deque queue = new ArrayDeque<>();
        while (!queue.isEmpty() || node != null) {
            while (node != null) {
                queue.push(node);
                node = node.left;
            }
            node = queue.pop();
            res.add(node.val);
            node = node.right;
        }
        return res;
    }

    //二叉树后续遍历
    public List postorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Deque stack = new ArrayDeque<>();
        TreeNode prev = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.right == null || root.right == prev) {
                res.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }
        }
        return res;
    }

}