0668. Kth Smallest Number in Multiplication Table

https://leetcode.com/problems/kth-smallest-number-in-multiplication-table

Description

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 * 104

• 1 <= k <= m * n

ac

class Solution {
    public int findKthNumber(int m, int n, int k) {
        // edge cases

        int l = 1, r = m * n;
        while (l < r) {
            int mid = l + (r - l) / 2;
            int cnt = countLessOrEqual(m, n, mid);
            // determine k
            if (cnt < k) l = mid + 1;
            else r = mid;
        }
        return l;
    }

    private int countLessOrEqual(int m, int n, int target) {
        // count less
        int less = 0,  c = n - 1;
        for (int r = 0; r < m; r++) {
            while (c >= 0 && (r+1) * (c+1) > target) c--;
            less += c + 1;
        }
        return less;
    }
}

/*
1) priority queue, O(k*mlogm), TLE;  2) binary search, O(m∗log(m∗n))
*/