题目链接
给你一棵二叉树的根节点 root ，返回其节点值的 后序遍历 。

示例 1：

输入：root = [1,null,2,3]
输出：[3,2,1]

示例2：

输入：root = []
输出：[]

示例3：

输入：root = [1]
输出：[1]

提示：

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

递归

/**
* 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 {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        postorder(root, result);
        return result;
    }

    private void postorder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        postorder(root.left, result);
        postorder(root.right, result);
        result.add(root.val);
    }
}

非递归

/**
 * 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 {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root == null) {
            return result;
        }
        stack.push(root);
        // 中->左->右入栈，出栈是中->右->左，翻转成左->右->中
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        Collections.reverse(result);
        return result;
    }
}

统一迭代

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> result = new ArrayList<>();

        if (root == null) {
            return result;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.peek();
            if (node != null) {
                stack.pop();
                stack.push(node);
                stack.push(null);
                if (node.right != null) {
                    stack.push(node.right);
                }
                if (node.left != null) {
                    stack.push(node.left);
                }
            } else {
                stack.pop();
                result.add(stack.pop().val);
            }
        }
        return result;
    }
}