We know that 4 and 7 are lucky digits. Also, a number is called lucky if it contains only lucky digits.
You are given an integer k, return the kth lucky number represented as a string.
Example 1:
Input: k = 4 Output: "47" Explanation: The first lucky number is 4, the second one is 7, the third one is 44 and the fourth one is 47.
Example 2:
Input: k = 10 Output: "477" Explanation: Here are lucky numbers sorted in increasing order: 4, 7, 44, 47, 74, 77, 444, 447, 474, 477. So the 10th lucky number is 477.
Example 3:
Input: k = 1000 Output: "777747447" Explanation: It can be shown that the 1000th lucky number is 777747447.
Constraints:
1 <= k <= 109