题解 | 二叉树的中序遍历

递归

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 {
    public void inorder(ArrayList<Integer> list,TreeNode root){
        if(root==null){
            return ;
        }
        inorder(list,root.left);
        list.add(root.val);
        inorder(list,root.right);

    }
    /**
     * 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
        ArrayList<Integer> list=new ArrayList<>();
        inorder(list,root);
        int[] res=new int[list.size()];
        res=list.stream().mapToInt(Integer::valueOf).toArray();//ArrayList转化为int[]
    
        return res;

    }
}