题解 | #农场牛的最佳观赏次序#

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
     *
     * 升序遍历，即中序遍历
     * @param root TreeNode类
     * @return int整型一维数组
     */
    public int[] inorderTraversal (TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            if (null != root) {
                stack.push(root);
                root = root.left;
            } else {
                TreeNode node = stack.pop();
                list.add(node.val);
                root = node.right;
            }
        }
        int[] arr = new int[list.size()];
        for (int i = 0; i < arr.length; i++ ) {
            arr[i] = list.get(i);
        }
        return arr;
    }
}