Description

Bisect on a BST finds the boundary near a target instead of an exact hit. Same idea as Bisect Left-Right, but you walk the tree instead of indexing an array.
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Reach for when

You want the closest value to a target in a BST (floor/ceiling/next/prev) not necessarily an exact match.

Runtime

balanced: O(log(n))
unbalanced: O(n)

Visualization

Pseudocode

Ceiling(node, target):        # smallest value >= target
	best = null
	while node:
		if node.value < target:    # too small → go right
			node = node.right
		else:                      # candidate → record, go left
			best = node.value
			node = node.left
	return best
# floor / bisect_right: swap  <  for  <=

Code

def bisect_left_bst(root, target):    # smallest value >= target (ceiling)
	best = None
	node = root
	while node:
		if node.val < target:         # too small → go right
			node = node.right
		else:                         # >= target → candidate, go left
			best = node.val
			node = node.left
	return best