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103. Binary Tree Zigzag Level Order Traversal 👍

Approach 1: Deque

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

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

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

    return ans;
  }
};
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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<>(Arrays.asList(root));
    boolean isLeftToRight = true;

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

    return ans;
  }
}
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class Solution:
  def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
    if not root:
      return []

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

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

    return ans

Approach 2: Queue

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
    if (!root)
      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(node);
      isLeftToRight = !isLeftToRight;
    }

    return ans;
  }
};
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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<>(Arrays.asList(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;
  }
}
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class Solution:
  def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
    if not root:
      return []

    ans = []
    q = 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