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) {
        // write code here
        List<Integer> intList = new ArrayList<>();

        midOrder(root, intList);

        int[] target = intList.stream().mapToInt(i -> i).toArray();
        return target;
    }

    private void midOrder(TreeNode root, List<Integer> intList){
        if(root != null){
            TreeNode left = root.left;
            TreeNode right = root.right;
            midOrder(left, intList);
            intList.add(root.val);
            midOrder(right, intList);
        }
    }

}