题目描述

给定一个二叉树，返回后序遍历的结果。

样例

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [3,2,1]

算法1

(Iterative) $O(n)$

iterative solution with recursive thinking.
credit to: https://leetcode.com/problems/binary-tree-postorder-traversal/discuss/45556/Java-simple-and-clean

JAVA 代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Deque<TreeNode> stk = new ArrayDeque<>();
        LinkedList<Integer> ans = new LinkedList<Integer>();
        if (root == null) return ans;
        stk.push(root);

        while (!stk.isEmpty()) {
            TreeNode cur = stk.pop();
            ans.addFirst(cur.val);
            if (cur.left != null) stk.push(cur.left);
            if (cur.right != null) stk.push(cur.right);
        }
        return  ans;

    }
}