Binary Tree Preorder Traversal

Solution 1: recursive

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root==null) return res;
        preOrder(root, res);
        return res;
    }

    private void preOrder(TreeNode root, List<Integer> res){
        if(root==null) return;
        res.add(root.val);
        preOrder(root.left, res);
        preOrder(root.right, res);
    }
}

Solution 2: iterative

基本思路：能用递归的用非递归写用stack

public class Solution{
    public List<Integer> preorderTraversal(TreeNode root){
        Stack<TreeNode> stack = new Stack<TreeNode>();
        List<Integer> preorder = new ArrayList<Integer>();

        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;
    }
}