Leetcode——264. Ugly Number II

题目

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, and n does not exceed 1690.

Hint:

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.Show More Hint

解答

class Solution {
public:
    int nthUglyNumber(int n) {
        int index[]={0,0,0};
        int primes[]={2,3,5};
        int *ugly=new int[n];

        for(int i=1;i<n;i++)
            ugly[i]=INT_MAX;
        ugly[0]=1;
        for(int i=0;i<n;i++)
        {
            for(int j=0;j<3;j++) ugly[i]=min(ugly[i],ugly[index[j]]*primes[j]);
            for(int j=0;j<3;j++) index[j]+=(ugly[i]==ugly[index[j]]*primes[j]);
        }

        return ugly[n-1];
    }
};