leetcode: Binary Tree Inorder Traversal

中序遍历一棵树，按照左子->根->右子的顺序遍历即可

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public ArrayList<Integer> inorderTraversal(TreeNode root) {        ArrayList<Integer> res = new ArrayList<Integer>();        res.clear();        if( root == null )        {            return res;        }        res = fun( root,res);        return res;    }    ArrayList<Integer> fun(TreeNode root,ArrayList<Integer> res)    {        if( root == null )        {            return res;        }        if( root.left != null )        {            fun(root.left,res);        }        res.add(root.val);        if( root.right != null )        {            fun(root.right,res);        }        return res;    }    }