递归

public class Solution {    public ArrayList
     
       preorderTraversal(TreeNode root) {        ArrayList
      
        res = new ArrayList
       
        ();        traversal(root, res);        return res;    }        public void traversal(TreeNode root, ArrayList
        
          res) {        if (root == null) {            return;        }                res.add(root.val);        traversal(root.left, res);        traversal(root.right, res);    }}
        
       
      
     

非递归

public class Solution {    public ArrayList
     
       preorderTraversal(TreeNode root) {        Stack
      
        stack = new Stack
       
        ();        ArrayList
        
          preorder = new ArrayList
         
          ();                if (root == null) {            return preorder;        }                stack.push(root);                while (!stack.isEmpty()) {            TreeNode node = stack.pop();            preorder.add(node.val);            if (node.right != null) {                stack.push(node.right);            }            if (node.left != null) {                stack.push(node.left);            }        }        return preorder;    }}
         
        
       
      
     

divide and conquer

public ArrayList
     
       preorderTraversal(TreeNode root) {        ArrayList
      
        res= new ArrayList
       
        ();        if (root == null) {            return res;        }                ArrayList
        
          left = preorderTraversal(root.left);        ArrayList
         
           right = preorderTraversal(root.right);                res.add(root.val);        res.addAll(left);        res.addAll(right);        return res;    }