leetcode: Binary Tree Preorder Traversal

非常基础的一道中序遍历二叉树的题目。递归的按照当前节点、左子结点、右子结点的顺序访问加入即可。

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