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
        if (root == null) return new int[0];
        List<Integer> list = new ArrayList<>();
        int[] res;
        midOrder(list, root);
        res = new int[list.size()];
        for (int i = 0; i < res.length; i++) res[i] = list.get(i);
        return res;
    }

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