Leetcode[313] Super Ugly Number

Leetcode[313] Super Ugly Number

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.

滚动求最大值

复杂度
考虑一个index，i
primes[index] * res[index[i]]的值是下一个可能的替补值。

思路
int[] res数组中保存的是之前保留到的值，因为下一个可能的值是primes[i]和之前的值的倍数关系。

代码

public int nthSuperUglyNumber(int n, int[] primes) {
    int[] res = new int[n];
    res[0] = 1;
    int[] index = new int[primes.length];
    for(int i = 1; i < n; i ++) {
        res[i] = Integer.MAX_VALUE;
        for(int j = 0; j < primes.length; j ++) {
            res[i] = Math.min(res[i], primes[j] * res[index[j]]);
        }
        //
        for(int j = 0; j < primes.length; j ++) {
            if(primes[j] * res[index[j]] == res[i]) {
                index[j] ++;
            }
        }
    }
    return res[n - 1];
}