二叉树的前序遍历

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: Preorder in ArrayList which contains node values.
*/
public ArrayList<Integer> preorderTraversal(TreeNode root) {
// write your code here
ArrayList<Integer> R = new ArrayList<Integer>();
preorder(root, R);

return R;
}

public int preorder(TreeNode root, ArrayList result)
{

if(root != null)
result.add(root.val);
else
return 0;

preorder(root.left, result);
preorder(root.right, result);

return 0;
}
}