## What is it? Given a sorted array and a target, return the index where the target would be inserted to maintain sorted order. If the target exists, return its index. This is the "lower bound" operation. ## How it works - Standard binary search but the result is `low` (not -1) when the target is not found - If `arr[mid] < target` → move `low = mid + 1` - If `arr[mid] >= target` → move `high = mid - 1` - At the end, `low` is the insertion position ## When to use - Inserting into a sorted array/list while maintaining order - Finding the leftmost position where a condition becomes true - Count of elements less than target ## Key Points - Returns index in range `[0, n]` — n means insert at the very end - Equivalent to `std::lower_bound` in C++ and `bisect_left` in Python - The "left boundary" binary search template