binary search

```
class Solution {
public:
int findKthNumber(int m, int n, int k) {
int l = 1, r = m*n, mid, res;
while(l<=r)
{
mid = (l+r) >> 1;
int cnt = 0;
for(int i = 1; i <= m; i++) cnt += min(mid / i, n);
if(cnt >= k)
{
res = mid;
r = mid - 1;
}
else if(cnt < k)
l = mid + 1;
}
return res;
}
};
```