103. Binary Tree Zigzag Level Order Traversal
Đề Bài
Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).
Example 1:
Input: root = [3,9,20,null,null,15,7] Output: [[3],[20,9],[15,7]]
Example 2:
Input: root = [1] Output: [[1]]
Example 3:
Input: root = [] Output: []
Constraints:
- The number of nodes in the tree is in the range
[0, 2000]. -100 <= Node.val <= 100
Thuật Toán & Kỹ Thuật
⏱️ Thời gian
O(n²)
💾 Không gian
O(n)
Lời Giải
Python
0103-binary-tree-zigzag-level-order-traversal.py
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
if not root:
return []
dq = deque([root])
level = 0
ans = []
while dq:
n = len(dq)
tmp = [0 for _ in range(n)]
for i in range(n):
node = dq.popleft()
if level:
tmp[n - i - 1] = node.val
else:
tmp[i] = node.val
if node.left:
dq.append(node.left)
if node.right:
dq.append(node.right)
level = not level
ans.append(tmp)
return ans