Numbers Made of Fours and Sevens

Medium

Consider the positive numbers whose decimal form uses only the digits 4 and 7: 4, 7, 44, 47, 74, 77, 444, and so on. Return the k-th smallest such number (counting from one) as a string.

Example 1

k=5 → '74'

In increasing order: 4, 7, 44, 47, 74, ... The fifth one is 74.

Example 2

k=2 → '7'

The second smallest is 7.

Example 3

k=14 → '777'

Six numbers have one or two digits, then come eight three-digit ones; the 14th is the last of those, 777.

Constraints

1 ≤ k ≤ 10¹⁵

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Topics

Bit Manipulation Math String

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Input

k=5

Expected

'74'

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