[LintCode] 518 Super Ugly Number 解题报告

Description
Write a program to find the nth super ugly number.

Super ugly numbers are positive numbers whose all prime factors are in the given prime list primes of size k. For example, [1, 2, 4, 7, 8, 13, 14, 16, 19, 26, 28, 32] is the sequence of the first 12 super ugly numbers given primes = [2, 7, 13, 19] of size 4.


Notice
1. 1 is a super ugly number for any given primes.
2. The given numbers in primes are in ascending order.
3. 0 < k ≤ 100, 0 < n ≤ 10^6, 0 < primes[i] < 1000


Example
Given n = 6, primes = [2, 7, 13, 19] return 13


思路
和Ugly Number II一模一样的做法。只不过扩展到每次需要检查k个list的最小值。


Code

public class Solution {
    /**
     * @param n a positive integer
     * @param primes the given prime list
     * @return the nth super ugly number
     */
    public int nthSuperUglyNumber(int n, int[] primes) {
        // Write your code here
        
        int[] times = new int[primes.length];
        int[] uglys = new int[n];
        uglys[0] = 1;
        
        for (int i = 1; i < n; i++) {
            int min = Integer.MAX_VALUE;
            for (int j = 0; j < primes.length; j++) {
                min = Math.min(min, primes[j] * uglys[times[j]]);
            }
            uglys[i] = min;
            
            for (int j = 0; j < times.length; j++) {
                if (uglys[times[j]] * primes[j] == min) {
                    times[j]++;
                }
            }
        }
        
        return uglys[n - 1];
    }
}