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Given an integer n
, return the nth
ugly number.
Ugly number is a positive number whose prime factors only include 2
, 3
, and/or 5
.
Example 1:
Input: n = 10
Output: 12
Explanation: [1, 2, 3, 4, 5, 6, 8, 9, 10, 12] is the sequence of the first 10 ugly numbers.
Example 2:
Input: n = 1
Output: 1
Explanation: 1 is typically treated as an ugly number.
Constraints:
[Heap]
[Math]
[Dynamic Programming]
Similar Questions
- Merge k Sorted Lists (Hard)
- Count Primes (Easy)
- Ugly Number (Easy)
- Perfect Squares (Medium)
- Super Ugly Number (Medium)
Hints
Hint 1
The naive approach is to call isUgly
for every number until you reach the nth one. Most numbers are not ugly. Try to focus your effort on generating only the ugly ones.
Hint 2
An ugly number must be multiplied by either 2, 3, or 5 from a smaller ugly number.
Hint 3
The key is how to maintain the order of the ugly numbers. Try a similar approach of merging from three sorted lists: L1, L2, and L3.
Hint 4
Assume you have Uk, the kth ugly number. Then Uk+1 must be Min(L1 * 2, L2 * 3, L3 * 5).