二叉树中序遍历

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(root==null)
            return list;
        inorder(root,list);
        return list;
    }
    public static void inorder(TreeNode root,ArrayList list){
        if(root!=null){
            inorder(root.left,list);
            list.add(root.val);
            inorder(root.right,list);
        }            
    }
}