题解 | #二叉树的中序遍历# 大神的想法

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 {

    private static ArrayList<Integer> list = new ArrayList<>();
    private static int i = 0;
    /**
     * 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[] inorderTraversal (TreeNode root) {
        // write code here


        zhongxu(root);
        int[] ints1 = list.stream().mapToInt(Integer::intValue).toArray();
        return ints1;
    }

    /**
         * 中序遍历
         * @param node
         */
    public static void zhongxu(TreeNode node) {
        if (node != null) {
            Stack<TreeNode> stack = new Stack<>();
            while (!stack.isEmpty() || node != null) {
                if (node != null) {
                    stack.push(node);
                    node = node.left;
                } else {
                    node = stack.pop();
                    System.out.println(node.val + "");
                    list.add(node.val);
                    node = node.right;
                }
            }
        }
        System.out.println();
    }

}