144. Binary Tree Preorder Traversal

Reference to 94

2ND TRY
100.00 %
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
Deque<TreeNode> deque = new ArrayDeque<>();
TreeNode curr = root;
while (curr != null || !deque.isEmpty()) {
while (curr != null) {
result.add(curr.val);
deque.addFirst(curr);
curr = curr.left;
}
curr = deque.removeFirst();
curr = curr.right;
}
return result;
    }
}

1ST TRY
/**
 * 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>();
        TreeNode curr = root;
        stack.push(root);
        while (!stack.isEmpty()) {
            curr = stack.pop();
            result.add(curr.val);
            if (curr.right != null) stack.push(curr.right);
            if (curr.left != null) stack.push(curr.left);
        }
        return result;
    }
}