#144 Binary Tree Preorder Treaversal.java

//https://leetcode.com/problems/binary-tree-preorder-traversal/
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
// Recursive
class Solution {
    List<Integer> res;
    public List<Integer> preorderTraversal(TreeNode root) {
        res = new LinkedList<>();
        preorder(root);
        return res;
    }
    public void preorder(TreeNode root) {
        if(root == null) return;
        res.add(root.val);
        preorder(root.left);
        preorder(root.right);
        return;
    }
}
//Iterative 
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> st = new Stack<>();
        List<Integer> res = new LinkedList<>();
        while(!st.isEmpty() || root != null) {
            while(root != null) {
                res.add(root.val);
                st.push(root);
                root = root.left;
            }
            root = st.pop();
            root = root.right;
        }
        return res;
    }
}