144_preorderTraversal.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        
        while(!stack.isEmpty()){
            TreeNode temp = stack.pop();
            result.add(temp.val);
            if(temp.right!=null){
                stack.push(temp.right);
            }
            if(temp.left!=null){
                stack.push(temp.left);
            }
        }
        return result;
        
    }
}

Recursive:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    List<Integer> result = new ArrayList<Integer>();
    
    public List<Integer> preorderTraversal(TreeNode root) {
        preorder(root);
        return result;
    }
    
    public void preorder(TreeNode root){
        if(root == null) return;
        result.add(root.val);
        preorder(root.left);
        preorder(root.right);
    }
}