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144. Binary Tree Preorder Traversal 👍

Approach 1: Recursive

  • Time: $O(n)$
  • Space: $O(h)$
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class Solution {
 public:
  vector<int> preorderTraversal(TreeNode* root) {
    vector<int> ans;
    preorder(root, ans);
    return ans;
  }

 private:
  void preorder(TreeNode* root, vector<int>& ans) {
    if (root == nullptr)
      return;

    ans.push_back(root->val);
    preorder(root->left, ans);
    preorder(root->right, ans);
  }
};

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

  private void preorder(TreeNode root, List<Integer> ans) {
    if (root == null)
      return;

    ans.add(root.val);
    preorder(root.left, ans);
    preorder(root.right, ans);
  }
}

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class Solution:
  def preorderTraversal(self, root: TreeNode | None) -> list[int]:
    ans = []

    def preorder(root: TreeNode | None) -> None:
      if not root:
        return

      ans.append(root.val)
      preorder(root.left)
      preorder(root.right)

    preorder(root)
    return ans

Approach 2: Iterative

  • Time: $O(n)$
  • Space: $O(h)$
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class Solution {
 public:
  vector<int> preorderTraversal(TreeNode* root) {
    if (root == nullptr)
      return {};

    vector<int> ans;
    stack<TreeNode*> stack{{root}};

    while (!stack.empty()) {
      root = stack.top(), stack.pop();
      ans.push_back(root->val);
      if (root->right)
        stack.push(root->right);
      if (root->left)
        stack.push(root->left);
    }

    return ans;
  }
};

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class Solution {
  public List<Integer> preorderTraversal(TreeNode root) {
    if (root == null)
      return new ArrayList<>();

    List<Integer> ans = new ArrayList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    stack.push(root);

    while (!stack.isEmpty()) {
      root = stack.pop();
      ans.add(root.val);
      if (root.right != null)
        stack.push(root.right);
      if (root.left != null)
        stack.push(root.left);
    }

    return ans;
  }
}

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class Solution:
  def preorderTraversal(self, root: TreeNode | None) -> list[int]:
    if not root:
      return []

    ans = []
    stack = [root]

    while stack:
      node = stack.pop()
      ans.append(node.val)
      if node.right:
        stack.append(node.right)
      if node.left:
        stack.append(node.left)

    return ans