Binary Tree Preorder Traversal (Iterative)

Easy Trees & BST Recursion Original on LeetCode

Binary Tree Preorder Traversal (Iterative) is a easy trees & bst problem solved with the recursion pattern. The best approach, iterative (stack), runs in O(n) time and O(h) space. Below are 2 approaches in Java, from recursive (reference) up.

Problem

Return the preorder traversal of a binary tree iteratively, using an explicit stack.

Examples

Example 1

Input: root = [5, 3, 8, 1, 4]
Output: [5, 3, 1, 4, 8]

Example 2

Input: root = []
Output: []

Constraints

The tree has 0 to 100 nodes.
Solve it without recursion.

Animated walkthrough

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Solutions

Try it yourself first. Then compare: each approach lists its idea, the steps, its time and space, and the Java code.

Approach	Time	Space
Recursive (reference)	`O(n)`	`O(h)`
Iterative (stack)	`O(n)`	`O(h)`

1Recursive (reference)

TimeO(n)

SpaceO(h)

Node, left, right with the call stack.

add; preorder(left); preorder(right).

Java

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> out = new ArrayList<>();
        go(root, out);
        return out;
    }

    private void go(TreeNode n, List<Integer> out) {
        if (n == null) return;
        out.add(n.val);
        go(n.left, out);
        go(n.right, out);
    }
}

2Iterative (stack)

TimeO(n)

SpaceO(h)

Pop a node and record it immediately. Push right then left, so the left subtree is fully processed before the right.

Push root.
Pop, add, push right (if any), push left (if any).

Java

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> out = new ArrayList<>();
        if (root == null) return out;
        Deque<TreeNode> st = new ArrayDeque<>();
        st.push(root);
        while (!st.isEmpty()) {
            TreeNode n = st.pop();
            out.add(n.val);
            if (n.right != null) st.push(n.right);
            if (n.left != null) st.push(n.left);
        }
        return out;
    }
}

Edge cases to test

Empty tree

Hints

Hint 1

A stack is last-in-first-out, so push the right child before the left one.

FAQ

What is the best time complexity for Binary Tree Preorder Traversal (Iterative)?

Iterative (stack) runs in O(n) time and O(h) extra space.

Which pattern does Binary Tree Preorder Traversal (Iterative) use?

It is a trees & bst problem that uses the recursion pattern. Other problems with the same pattern: Binary Tree Inorder Traversal (Recursive), Binary Tree Preorder Traversal (Recursive), Binary Tree Postorder Traversal (Recursive).

Is there a brute force solution for Binary Tree Preorder Traversal (Iterative)?

Yes. Recursive (reference) takes O(n) time and O(h) space. Node, left, right with the call stack.

Which edge cases should I test for Binary Tree Preorder Traversal (Iterative)?

Empty tree.