题解 | #二叉树的后序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
        if (root == null)
            return new int[0];
        Stack<TreeNode> stack = new Stack<TreeNode>();
        List<Integer> lists = new ArrayList<Integer>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            TreeNode node = stack.pop();
            if (node.right == null || node.right == pre) {
                lists.add(node.val);
                pre = node;
            } else {
                stack.push(node);
                root = node.right;
            }
        }
        int[] res = new int[lists.size()];
        for (int i = 0; i < res.length; i++)
            res[i] = lists.get(i);
        return res;
    }
}