lintcode,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) {
ArrayList<Integer> res = new ArrayList<Integer>();
if(root == null) return res;
res.add(root.val);
if(root.left != null) res.addAll(preorderTraversal(root.left));
if(root.right != null) res.addAll(preorderTraversal(root.right));
return res;

}
}

/**
* 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) {
ArrayList<Integer> res = new ArrayList<Integer>();
if(root == null) return res;
Stack<TreeNode> stack = new Stack<TreeNode>();
stack.push(root);
while(!stack.empty()){
TreeNode node = stack.pop();
res.add(node.val);
if(node.right != null) stack.push(node.right);
if(node.left != null) stack.push(node.left);
}
return res;
}
}