378. Kth Smallest Element in a Sorted Matrix

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matrix is an n x n grid where every row and every column is sorted in non-decreasing order. Return the k-th smallest value among all n^2 entries, counting duplicates: if the sorted list of all entries is v, return v[k - 1].

Try to use less than O(n^2) extra memory.

Example

matrix = [[1,5,9],
          [10,11,13],
          [12,13,15]], k = 8  ->  13

All entries in order are 1,5,9,10,11,12,13,13,15; the 8th is 13.

Constraints