Date: 2026-06-06
Time: 18:00
This file solves LeetCode 2379: Minimum Recolors to Get K Consecutive Black Blocks. Given a string of 'W' (white) and 'B' (black) characters, it finds the minimum number of white blocks you'd need to repaint black to create a contiguous run of k black blocks.
It owns exactly one responsibility: the min_operations function.
min_operations(blocks: str, k: int) -> intContract: Given a string blocks containing only 'W' and 'B', and an integer k where 1 <= k <= len(blocks), returns the minimum number of recolors (white-to-black) needed to produce k consecutive black blocks.
Fixed-size sliding window. This is the textbook application: maintain a count over a window of size k, slide it one position at a time, and track the minimum.
The implementation avoids recomputing the count from scratch each step. Instead, it updates incrementally by adding the incoming element and removing the outgoing one. The expression (blocks[i] == "W") - (blocks[i - k] == "W") exploits Python's bool-to-int coercion — True becomes 1, False becomes 0 — to do the add/remove in a single arithmetic statement.
Imports: None. Pure standard Python.
Imported by: The test_solution.py in the same directory. The "Imported By" list in the prompt is misleading — those are all unrelated test files for other problems, likely an artifact of the indexing tool matching on a shared test harness pattern, not actual imports of this module.
1. Initialize window (line 14): Count 'W's in blocks[0:k] using str.count. This is the cost of making the *first* window all-black.
2. Set baseline (line 15): minops = whitecount.
3. Slide (lines 18–19): For each position i from k to len(blocks) - 1:
blocks[i] is 'W' (entering the window).blocks[i - k] is 'W' (leaving the window).min_ops if the new count is lower.4. Return min_ops.
k elements. The loop index i is the right edge; i - k is the element just left of the window.white_count is always the exact number of 'W's in the current window blocks[i-k+1 : i+1].min_ops is monotonically non-increasing or stays the same — it never rises.None. The function trusts its inputs match the LeetCode constraints. If k > len(blocks), blocks[:k].count("W") still works (counts over the full string), but the loop body never executes, so you'd get the count of 'W's in the entire string — which would be a valid but semantically wrong answer for an invalid input. No validation is performed; this is typical for LeetCode solutions.
minimum-recolors-to-get-k-consecutive-black-blocks/test_solution.py — See the test cases and edge cases exercised against this solutiondiet-plan-performance/solution.py — Another fixed-size sliding window problem; compare the window update patternmaximum-average-subarray-i/solution.py — Classic sliding window over numeric arrays; contrast with this string-based variantsliding-window-variants — How fixed-size windows (this problem) differ from variable-size windows (e.g., minimum window substring)sliding-window-o1-update — min_operations updates the window count in O(1) per step by adding/removing single elements, not recomputing over the full windowbool-int-coercion-trick — The increment expression relies on Python's True == 1 / False == 0 to combine add and remove into one arithmetic operationno-input-validation — min_operations performs no validation on blocks content or the relationship between k and len(blocks)initial-window-uses-str-count — The first window's white count is computed via str.count("W") rather than the same incremental logic used for subsequent windows