题解 | #二叉树的中序遍历#

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[] inorderTraversal (TreeNode root) {

        ArrayList<Integer> result = new ArrayList<>();

        treeDeep(root, result);

        return result.stream().mapToInt(Integer :: valueOf).toArray();
    }

    public void treeDeep (TreeNode root, ArrayList<Integer> result) {

        if (root == null) {
            return;
        }

        treeDeep(root.left, result);
        result.add(root.val);
        treeDeep(root.right, result);
    }
}