คลาส TreeNode และ BFS แบบเรียงตามระดับ

พื้นฐานของคลาส TreeNode

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

# Build a small tree manually:
#       1
#      / \
#     2   3
#    / \
#   4   5
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
print(root.val, root.left.val, root.right.val)

การสร้างต้นไม้จากอาร์เรย์

from collections import deque

def build_tree(arr):
    if not arr or arr[0] is None:
        return None
    root = TreeNode(arr[0])
    q = deque([root])
    i = 1
    while q and i < len(arr):
        node = q.popleft()
        if i < len(arr) and arr[i] is not None:
            node.left = TreeNode(arr[i])
            q.append(node.left)
        i += 1
        if i < len(arr) and arr[i] is not None:
            node.right = TreeNode(arr[i])
            q.append(node.right)
        i += 1
    return root

root = build_tree([1, 2, 3, 4, 5, None, 6])
print(root.val, root.left.val, root.right.val)

BFS คืออะไรและเหตุใดจึงใช้คิว

from collections import deque

def bfs_print(root):
    if not root:
        return
    q = deque([root])
    while q:
        node = q.popleft()
        print(node.val, end=' ')
        if node.left:
            q.append(node.left)
        if node.right:
            q.append(node.right)

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
bfs_print(root)  # 1 2 3 4

BFS ตามระดับ: การจัดกลุ่มตามระดับ

from collections import deque

def level_order(root):
    if not root:
        return []
    result = []
    q = deque([root])
    while q:
        level_size = len(q)
        level = []
        for _ in range(level_size):
            node = q.popleft()
            level.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        result.append(level)
    return result

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
print(level_order(root))  # [[1], [2, 3], [4]]

ความลึกสูงสุดด้วย BFS

from collections import deque

def max_depth_bfs(root):
    if not root:
        return 0
    depth = 0
    q = deque([root])
    while q:
        depth += 1
        for _ in range(len(q)):
            node = q.popleft()
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
    return depth

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
print(max_depth_bfs(root))  # 3

มุมมองด้านขวาของต้นไม้ทวิภาค

from collections import deque

def right_side_view(root):
    if not root:
        return []
    result = []
    q = deque([root])
    while q:
        level_size = len(q)
        for i in range(level_size):
            node = q.popleft()
            if i == level_size - 1:
                result.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
    return result

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.right = TreeNode(5)
print(right_side_view(root))  # [1, 3, 5]

การท่องต้นไม้ตามระดับแบบซิกแซ็ก

from collections import deque

def zigzag_level_order(root):
    if not root:
        return []
    result = []
    q = deque([root])
    left_to_right = True
    while q:
        level = []
        for _ in range(len(q)):
            node = q.popleft()
            level.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        result.append(level if left_to_right else level[::-1])
        left_to_right = not left_to_right
    return result

root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20)
root.right.left = TreeNode(15)
root.right.right = TreeNode(7)
print(zigzag_level_order(root))

การวิเคราะห์ความซับซ้อนด้านพื้นที่ของ BFS

# Space comparison: BFS vs DFS on a complete binary tree
# n=15 nodes, height=4
# BFS max queue size = 8 (last level)
# DFS max call stack = 4 (height)

# For a skewed tree (like a linked list):
# n=1000 nodes
# BFS max queue size = 1 (always 1 node per level)
# DFS max call stack = 1000 (recursion depth -> stack overflow!)

from collections import deque

def skewed_tree(n):
    root = TreeNode(1)
    cur = root
    for i in range(2, n+1):
        cur.right = TreeNode(i)
        cur = cur.right
    return root

root = skewed_tree(10)
print('BFS on skewed tree is safe')

ค่าเฉลี่ยของแต่ละระดับในต้นไม้ทวิภาค

from collections import deque

def average_of_levels(root):
    if not root:
        return []
    result = []
    q = deque([root])
    while q:
        size = len(q)
        total = 0
        for _ in range(size):
            node = q.popleft()
            total += node.val
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        result.append(total / size)
    return result

root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20)
root.right.left = TreeNode(15)
root.right.right = TreeNode(7)
print(average_of_levels(root))  # [3.0, 14.5, 11.0]

ความลึกต่ำสุดด้วย BFS

from collections import deque

def min_depth(root):
    if not root:
        return 0
    q = deque([(root, 1)])
    while q:
        node, depth = q.popleft()
        # A leaf has no children
        if not node.left and not node.right:
            return depth
        if node.left:
            q.append((node.left, depth + 1))
        if node.right:
            q.append((node.right, depth + 1))
    return 0

root = TreeNode(2)
root.left = TreeNode(3)
root.left.left = TreeNode(4)
root.right = TreeNode(5)  # leaf at depth 2
print(min_depth(root))  # 2

การเชื่อมโยงโหนดพี่น้องตามระดับ

from collections import deque

class Node:
    def __init__(self, val=0, left=None, right=None, next=None):
        self.val = val
        self.left = left
        self.right = right
        self.next = next

def connect(root):
    if not root:
        return root
    q = deque([root])
    while q:
        size = len(q)
        for i in range(size):
            node = q.popleft()
            if i < size - 1:
                node.next = q[0]
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
    return root

print('BFS connect: O(n) time, O(w) space')

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