4. Binary Tree Preorder Traversal (Iterative)

Given the root of a binary tree, return the preorder traversal of its nodes' values.

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [1,2]

Example 5:

Input: root = [1,null,2]
Output: [1,2]

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Follow up:

Recursive solution is trivial, could you do it iteratively?

Solution

class Solution {
 public List<Integer> preorderTraversal(TreeNode root) {
 
 List<Integer> sol = new ArrayList<>();
 
 if(root == null)
 return sol;
 
 Stack<TreeNode> s = new Stack<>();
 s.push(root);
 
 while(!s.isEmpty()) {
 TreeNode top = s.pop();
 sol.add(top.val);
 if(top.right != null)
 s.push(top.right);
 if(top.left != null)
 s.push(top.left);
 }
 
 return sol;
 
 }
}