LeetcodeSuper Ugly Number

Posted 2020-07-10 wuezs

题目链接：https://leetcode.com/problems/super-ugly-number/

题目：
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.

Note:
(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 ≤ 106, 0 < primes[i] < 1000.

思路：
primes每个元素乘以当前res的结果 都一定会在之后的res之后出现。
用idx数组记录每个prime下一个应该相乘的res元素，每次计算res后更新idx。

算法：

 public int nthSuperUglyNumber(int n, int[] primes) {
        int idx[]=new int[primes.length];
        int res[]=new int[n];
        res[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]*res[idx[j]]);
            }
            res[i]=min;

            for(int k=0;k<primes.length;k++){
                if(min==res[idx[k]]*primes[k]){
                    idx[k]++;
                }

            }
        }
        return res[n-1];
    }