JAVA二叉树先序遍历递归+非递归

非递归先序遍历：

<span style="font-size:24px;">public ArrayList<Integer> preorderTraversal(TreeNode root) {
if(root==null)
return new ArrayList<Integer> ();
ArrayList<Integer> list=new ArrayList<Integer> ();
Stack<TreeNode> s=new Stack<TreeNode>();
TreeNode p=root;
while(p!=null||!s.isEmpty()){
while(p!=null){
list.add(p.val);
s.push(p);
p=p.left;
}
if(!s.isEmpty()){
p=s.pop();
p=p.right;
}
}
return list;
}</span>

递归先序遍历：

<span style="font-size:24px;">    ArrayList<Integer>list=new ArrayList<Integer>();
public ArrayList<Integer> preorderTraversal(TreeNode root){
if(root==null)
return list;
list.add(root.val);
preorderTraversal(root.left);
preorderTraversal(root.right);
return list;
}</span>