Binary Search — Kth Smallest in a Sorted Matrix

solve(matrix, k) — matrix is n × n and both its rows and its columns are sorted
ascending (unlike in Search a 2D Matrix, a row can start below the end of the row
above it). Return the k-th smallest
value, counting duplicates (k is 1-based).

A min-heap works in O(k log n). A tighter solution binary searches the value range
[matrix[0][0], matrix[n-1][n-1]]: for a candidate x, count how many cells are
<= x with a staircase walk from the bottom-left corner in O(n). The answer is the
smallest x whose count reaches k.

Examples

solve([[1,5,9],[10,11,13],[12,13,15]], 8)  → 13
solve([[-5]], 1)                           → -5
solve([[1,2],[1,3]], 3)                    → 2     // 1, 1, 2, 3