leetcode 668. Kth Smallest Number in Multiplication Table

Nearly every one have used the Multiplication Table. But could you find out the k-thsmallest number quickly from the multiplication table?

Given the height m and the length n of a m * n Multiplication Table, and a positive integer k, you need to return the k-th smallest number in this table.

Example 1:

Input: m = 3, n = 3, k = 5
Output: 
Explanation: 
The Multiplication Table:
1	2	3
2	4	6
3	6	9

The 5-th smallest number is 3 (1, 2, 2, 3, 3).

 

Example 2:

Input: m = 2, n = 3, k = 6
Output: 
Explanation: 
The Multiplication Table:
1	2	3
2	4	6

The 6-th smallest number is 6 (1, 2, 2, 3, 4, 6).

 

Note:

1. The m and n will be in the range [1, 30000].
2. The k will be in the range [1, m * n]

class Solution {
public:
    int findKthNumber(int m, int n, int k) 
    {
        int left = 1,right = m*n;
        int mid;
        while(left < right)
        {
            mid = left + (right - left)/2;
            int count = 0;
            int j=n;
            for(int i=1;i<=m;i++)
            {
                while(j>=1 && i*j>mid) j--;
                count += j;
            }
            if(count<k) left = mid+1;
            else
            {
                right = mid;
            }
        }
        return left;
    }
};