ค่าที่น้อยที่สุดลำดับ k ผลรวมช่วง และ BST เป็นอาร์เรย์เรียงลำดับ

องค์ประกอบลำดับที่ k ที่น้อยที่สุดใน BST

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

def kth_smallest(root, k):
    count = [0]
    result = [None]

    def inorder(node):
        if not node or result[0] is not None:
            return
        inorder(node.left)
        count[0] += 1
        if count[0] == k:
            result[0] = node.val
            return
        inorder(node.right)

    inorder(root)
    return result[0]

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

ค่าที่ k น้อยที่สุด: แบบวนซ้ำด้วยสแตก

def kth_smallest_iterative(root, k):
    stack = []
    curr = root
    count = 0
    while curr or stack:
        while curr:             # go as far left as possible
            stack.append(curr)
            curr = curr.left
        curr = stack.pop()      # process node
        count += 1
        if count == k:
            return curr.val
        curr = curr.right       # move to right subtree
    return -1  # k out of range

root = TreeNode(5)
root.left = TreeNode(3)
root.right = TreeNode(6)
root.left.left = TreeNode(2)
root.left.right = TreeNode(4)
root.left.left.left = TreeNode(1)
print(kth_smallest_iterative(root, 3))  # 3

องค์ประกอบลำดับที่ k ที่มากที่สุดใน BST

def kth_largest(root, k):
    count = [0]
    result = [None]

    def reverse_inorder(node):
        if not node or result[0] is not None:
            return
        reverse_inorder(node.right)   # visit LARGER values first
        count[0] += 1
        if count[0] == k:
            result[0] = node.val
            return
        reverse_inorder(node.left)

    reverse_inorder(root)
    return result[0]

root = TreeNode(3)
root.left = TreeNode(1)
root.right = TreeNode(4)
root.left.right = TreeNode(2)
print(kth_largest(root, 1))  # 4 (largest)
print(kth_largest(root, 2))  # 3 (2nd largest)

ผลรวมตามช่วงของ BST

def range_sum_bst(root, low, high):
    if not root:
        return 0
    total = 0
    if low <= root.val <= high:
        total += root.val
    if root.val > low:    # left subtree might have values >= low
        total += range_sum_bst(root.left, low, high)
    if root.val < high:   # right subtree might have values <= high
        total += range_sum_bst(root.right, low, high)
    return total

root = TreeNode(10)
root.left = TreeNode(5)
root.right = TreeNode(15)
root.left.left = TreeNode(3)
root.left.right = TreeNode(7)
root.right.right = TreeNode(18)
print(range_sum_bst(root, 7, 15))  # 7+10+15 = 32

นับโหนดในช่วง

def count_range(root, low, high):
    if not root:
        return 0
    count = 0
    if low <= root.val <= high:
        count += 1
    if root.val > low:
        count += count_range(root.left, low, high)
    if root.val < high:
        count += count_range(root.right, low, high)
    return count

root = TreeNode(10)
root.left = TreeNode(5)
root.right = TreeNode(15)
root.left.left = TreeNode(3)
root.left.right = TreeNode(7)
root.right.right = TreeNode(18)
print(count_range(root, 6, 15))  # 7, 10, 15 = 3

BST เป็นอาร์เรย์เรียงลำดับ (อัลกอริทึมฉบับเต็ม)

def bst_to_sorted(root):
    result = []
    def inorder(node):
        if not node:
            return
        inorder(node.left)
        result.append(node.val)
        inorder(node.right)
    inorder(root)
    return result

root = TreeNode(5)
root.left = TreeNode(3)
root.right = TreeNode(8)
root.left.left = TreeNode(1)
root.left.right = TreeNode(4)
root.right.left = TreeNode(6)
root.right.right = TreeNode(9)
print(bst_to_sorted(root))  # [1, 3, 4, 5, 6, 8, 9]

# Binary search on the resulting sorted array:
import bisect
arr = bst_to_sorted(root)
print(bisect.bisect_left(arr, 6))   # 4 (index of 6)

BST แบบเพิ่มข้อมูล: ขนาดทรีย่อย

class AugNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None
        self.size = 1  # subtree size

def get_size(node):
    return node.size if node else 0

def update_size(node):
    if node:
        node.size = 1 + get_size(node.left) + get_size(node.right)

def kth_smallest_aug(root, k):
    left_size = get_size(root.left)
    if k == left_size + 1:
        return root.val      # current node is kth
    elif k <= left_size:
        return kth_smallest_aug(root.left, k)
    else:
        return kth_smallest_aug(root.right, k - left_size - 1)

print('Augmented BST: O(log n) kth smallest with subtree sizes')

ค้นหาค่าทั้งหมดใน BST ระหว่างสองโหนด

def values_between(root, low, high):
    result = []
    def inorder(node):
        if not node:
            return
        if node.val > low:    # might be values > low on left
            inorder(node.left)
        if low < node.val < high:  # strictly between
            result.append(node.val)
        if node.val < high:   # might be values < high on right
            inorder(node.right)
    inorder(root)
    return result

root = TreeNode(10)
root.left = TreeNode(5)
root.right = TreeNode(15)
root.left.left = TreeNode(3)
root.left.right = TreeNode(7)
root.right.left = TreeNode(12)
root.right.right = TreeNode(18)
print(values_between(root, 6, 15))  # [7, 10, 12]

มัธยฐานของ BST

def count_nodes(root):
    if not root:
        return 0
    return 1 + count_nodes(root.left) + count_nodes(root.right)

def median_of_bst(root):
    n = count_nodes(root)
    if n == 0:
        return None
    k = n // 2 + 1  # (n+1)/2-th element for odd, n/2+1-th for even
    return kth_smallest(root, k)

def kth_smallest(root, k):
    count = [0]; result = [None]
    def inorder(node):
        if not node or result[0] is not None: return
        inorder(node.left)
        count[0] += 1
        if count[0] == k: result[0] = node.val; return
        inorder(node.right)
    inorder(root); return result[0]

root = TreeNode(5)
root.left = TreeNode(3)
root.right = TreeNode(8)
root.left.left = TreeNode(1)
root.left.right = TreeNode(4)
print(median_of_bst(root))  # 4 (middle of [1,3,4,5,8])

ค้นหา k ค่าที่ใกล้เป้าหมายที่สุด

import heapq

def closest_k_values(root, target, k):
    # Collect sorted values
    arr = []
    def inorder(node):
        if not node: return
        inorder(node.left)
        arr.append(node.val)
        inorder(node.right)
    inorder(root)

    # Two-pointer sliding window of size k
    left, right = 0, k - 1
    while right < len(arr) - 1:
        if abs(arr[left] - target) <= abs(arr[right + 1] - target):
            break  # left is closer, don't advance
        left += 1
        right += 1
    return arr[left:right + 1]

root = TreeNode(4)
root.left = TreeNode(2)
root.right = TreeNode(5)
root.left.left = TreeNode(1)
root.left.right = TreeNode(3)
print(closest_k_values(root, 3.7, 2))  # [3, 4]

ใช้ประโยชน์จากคุณสมบัติของลำดับค่าถัดไป

# Meta-pattern for BST problems:
# Step 1: What sorted-order property does this exploit?
# Step 2: Is in-order (ascending) or reverse in-order (descending) needed?
# Step 3: Can I prune using BST ordering to avoid O(n) scan?

# Quick reference:
# kth smallest  -> in-order, stop at kth node
# kth largest   -> reverse in-order, stop at kth node
# range sum     -> in-order + BST pruning
# closest value -> walk toward target, track best
# median        -> kth with k = n//2+1
# sorted array  -> full in-order
# validate      -> in-order prev check or min/max bounds
print('Sorted in-order is the universal BST problem tool')

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