## What is it? Find both the floor (largest element ≤ target) and ceil (smallest element ≥ target) of a target value in a sorted array in a single pass using binary search. ## How it works - Run binary search tracking two candidates: `floor` and `ceil` - If `arr[mid] == target` → floor = ceil = `arr[mid]`; done - If `arr[mid] < target` → `arr[mid]` is a floor candidate; update floor, move `low = mid + 1` - If `arr[mid] > target` → `arr[mid]` is a ceil candidate; update ceil, move `high = mid - 1` ## When to use - Finding the two closest values to a target in a sorted array - Interval queries and nearest-neighbor lookups - Prerequisite for more complex binary search problems ## Key Points - Both floor and ceil can be found in a single binary search pass - If target exists in the array, floor == ceil == target - If target is smaller than all elements, floor does not exist; if larger than all, ceil does not exist