144. Binary Tree Preorder Traversal

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class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        
        while (cur != null || !stack.isEmpty()) {
            while(cur != null) {
                res.add(cur.val);
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            cur = cur.right;
        }
        return res;

    }
}

new version of iteration

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

recursion

this is more clean

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }
    private void helper(TreeNode root, List<Integer> res) {
        if (root == null) return;
        res.add(root.val);
        helper(root.left, res);
        helper(root.right, res);
    }
}