103. Binary Tree Zigzag Level Order Traversal

Approach 1: Deque

• Time: $O(n)$
• Space: $O(n)$

C++JavaPython

class Solution {
 public:
  vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
    if (root == nullptr)
      return {};

    vector<vector<int>> ans;
    deque<TreeNode*> dq{{root}};
    bool isLeftToRight = true;

    while (!dq.empty()) {
      vector<int> currLevel;
      for (int sz = dq.size(); sz > 0; --sz)
        if (isLeftToRight) {
          TreeNode* node = dq.front();
          dq.pop_front();
          currLevel.push_back(node->val);
          if (node->left)
            dq.push_back(node->left);
          if (node->right)
            dq.push_back(node->right);
        } else {
          TreeNode* node = dq.back();
          dq.pop_back();
          currLevel.push_back(node->val);
          if (node->right)
            dq.push_front(node->right);
          if (node->left)
            dq.push_front(node->left);
        }
      ans.push_back(currLevel);
      isLeftToRight = !isLeftToRight;
    }

    return ans;
  }
};

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

    List<List<Integer>> ans = new ArrayList<>();
    Deque<TreeNode> dq = new ArrayDeque<>(List.of(root));
    boolean isLeftToRight = true;

    while (!dq.isEmpty()) {
      List<Integer> currLevel = new ArrayList<>();
      for (int sz = dq.size(); sz > 0; --sz)
        if (isLeftToRight) {
          TreeNode node = dq.pollFirst();
          currLevel.add(node.val);
          if (node.left != null)
            dq.addLast(node.left);
          if (node.right != null)
            dq.addLast(node.right);
        } else {
          TreeNode node = dq.pollLast();
          currLevel.add(node.val);
          if (node.right != null)
            dq.addFirst(node.right);
          if (node.left != null)
            dq.addFirst(node.left);
        }
      ans.add(currLevel);
      isLeftToRight = !isLeftToRight;
    }

    return ans;
  }
}

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

    ans = []
    dq = collections.deque([root])
    isLeftToRight = True

    while dq:
      currLevel = []
      for _ in range(len(dq)):
        if isLeftToRight:
          node = dq.popleft()
          currLevel.append(node.val)
          if node.left:
            dq.append(node.left)
          if node.right:
            dq.append(node.right)
        else:
          node = dq.pop()
          currLevel.append(node.val)
          if node.right:
            dq.appendleft(node.right)
          if node.left:
            dq.appendleft(node.left)
      ans.append(currLevel)
      isLeftToRight = not isLeftToRight

    return ans

Approach 2: Queue

• Time: $O(n)$
• Space: $O(n)$

C++JavaPython

class Solution {
 public:
  vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
    if (root == nullptr)
      return {};

    vector<vector<int>> ans;
    queue<TreeNode*> q{{root}};
    bool isLeftToRight = true;

    while (!q.empty()) {
      const int size = q.size();
      vector<int> currLevel(size);
      for (int i = 0; i < size; ++i) {
        TreeNode* node = q.front();
        q.pop();
        const int index = isLeftToRight ? i : size - i - 1;
        currLevel[index] = node->val;
        if (node->left)
          q.push(node->left);
        if (node->right)
          q.push(node->right);
      }
      ans.push_back(currLevel);
      isLeftToRight = !isLeftToRight;
    }

    return ans;
  }
};

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

    List<List<Integer>> ans = new ArrayList<>();
    Deque<TreeNode> q = new ArrayDeque<>(List.of(root));
    boolean isLeftToRight = true;

    while (!q.isEmpty()) {
      List<Integer> currLevel = new ArrayList<>();
      for (int sz = q.size(); sz > 0; --sz) {
        TreeNode node = q.poll();
        if (isLeftToRight)
          currLevel.add(node.val);
        else
          currLevel.add(0, node.val);
        if (node.left != null)
          q.offer(node.left);
        if (node.right != null)
          q.offer(node.right);
      }
      ans.add(currLevel);
      isLeftToRight = !isLeftToRight;
    }

    return ans;
  }
}

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

    ans = []
    q = collections.deque([root])
    isLeftToRight = True

    while q:
      size = len(q)
      currLevel = [0] * size
      for i in range(size):
        node = q.popleft()
        index = i if isLeftToRight else size - i - 1
        currLevel[index] = node.val
        if node.left:
          q.append(node.left)
        if node.right:
          q.append(node.right)
      ans.append(currLevel)
      isLeftToRight = not isLeftToRight

    return ans