668. Kth Smallest Number in Multiplication Table 
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Problem
Nearly everyone has used the Multiplication Table. The multiplication table of size m x n is an integer matrix mat where mat[i][j] == i * j (1-indexed).
Given three integers m, n, and k, return the kth smallest element in the m x n multiplication table.
Example 1:
Input: m = 3, n = 3, k = 5 Output: 3 Explanation: The 5th smallest number is 3.
Example 2:
Input: m = 2, n = 3, k = 6 Output: 6 Explanation: The 6th smallest number is 6.
Constraints:
1 <= m, n <= 3 * 1041 <= k <= m * n