How hard is LeetCode 2802. Find The K-th Lucky Number?

LeetCode 2802. Find The K-th Lucky Number is rated Medium on LeetCode.

What is the time complexity of LeetCode 2802. Find The K-th Lucky Number?

The Python solution on this page runs in O(\log k).

What is the space complexity of LeetCode 2802. Find The K-th Lucky Number?

The Python solution on this page uses O(\log k) auxiliary space.

What topics does LeetCode 2802. Find The K-th Lucky Number cover?

LeetCode 2802. Find The K-th Lucky Number is tagged Bit Manipulation, Math and String on LeetCode.

Is LeetCode 2802. Find The K-th Lucky Number a premium problem?

Yes. LeetCode 2802. Find The K-th Lucky Number is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.

Find The K-th Lucky Number — LeetCode 2802 Python Solution

MediumLeetCode PremiumBit ManipulationMathString

Problem: #2802
Pattern: Bit Manipulation
Reading time: 3 min
Source: leetcode.com

The problem

We know that 4 and 7 are lucky digits. Also, a number is called lucky if it contains only lucky digits.

Example

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.

Python solution

Python

class Solution:
    def kthLuckyNumber(self, k: int) -> str:
        n = 1
        while k > 1 << n:
            k -= 1 << n
            n += 1
        ans = []
        while n:
            n -= 1
            if k <= 1 << n:
                ans.append("4")
            else:
                ans.append("7")
                k -= 1 << n
        return "".join(ans)

Complexity

Measure	Complexity
Time	O(\log k)
Space	O(\log k) auxiliary

Pattern: Bit Manipulation

Use XOR, masks and the low-bit trick to replace whole data structures with an integer. LeetCode 2802. Find The K-th Lucky Number is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Bit Manipulation.

The bit manipulation guide has the Python template for the pattern and the 194 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 2802. Find The K-th Lucky Number?: LeetCode 2802. Find The K-th Lucky Number is rated Medium on LeetCode.
What is the time complexity of LeetCode 2802. Find The K-th Lucky Number?: The Python solution on this page runs in O(\log k).
What is the space complexity of LeetCode 2802. Find The K-th Lucky Number?: The Python solution on this page uses O(\log k) auxiliary space.
What topics does LeetCode 2802. Find The K-th Lucky Number cover?: LeetCode 2802. Find The K-th Lucky Number is tagged Bit Manipulation, Math and String on LeetCode.
Is LeetCode 2802. Find The K-th Lucky Number a premium problem?: Yes. LeetCode 2802. Find The K-th Lucky Number is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.