668 - binary search

class Solution:
    def findKthNumber(self, m: int, n: int, k: int) -> int:

        l = 0
        r = m * n

        def check(x):
            i = n
            j = 1
            count = 0
            while i > 0 and j <= m:
                if i * j <= x:
                    count += i
                    j += 1
                else:
                    i -= 1
            return count


        while r - l > 1:
            mid = l + (r - l) // 2
            if check(mid) < k:
                l = mid
            else:
                r = mid

        return r