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Question

Write a program to find the n-th ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note that 1 is typically treated as an ugly number.

Hint:

1. The naive approach is to call isUgly for every number until you reach the $n^{th}$ one. Most numbers are not ugly. Try to focus your effort on generating only the ugly ones.
2. An ugly number must be multiplied by either 2, 3, or 5 from a smaller ugly number.
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.
4. Assume you have U_k, the $k^{th}$ ugly number. Then U_k+1 must be Min(L₁ * 2, L₂ * 3, L₃ * 5).

Solution

Result: Accepted Time: 4 ms

Here should be some explanations.

int data[2000]={0};

int nthUglyNumber(int n)
{

   if(!data[0])
   {
       int p2, p3, p5, v2, v3, v5, cnt, tmp;
       cnt = data[0] = 1;
       p2 = p3 = p5 = 0;
       while(cnt < 2000)
       {
           tmp = v2 = data[p2] * 2;
           v3 = data[p3] * 3;
           v5 = data[p5] * 5;
           if( tmp > v3 ) tmp = v3;
           if( tmp > v5 ) tmp = v5;
           p2 += v2 == tmp;
           p3 += v3 == tmp;
           p5 += v5 == tmp;
           data[cnt++] = tmp;
       }
   }
   return data[n - 1];
}

Complexity Analytics

• Time Complexity: $O(1)$
• Space Complexity: $O(1)$