力不从心 Leetcode(ugly number heap) 263, 264,313

Posted 2020-11-02 wz30

Leetcode ugly number set (3 now)

 new ugly number is generated by multiplying a prime with previous generated ugly numbe

Is ugly number or not

class Solution {
    public boolean isUgly(int num) {
        if(num==0) return false;
        //how to generate the number
        while(num%2==0) num/=2;
        while(num%3==0) num/=3;
        while(num%5==0) num/=5;
        return num==1;
    }
}

//simulate the process to generate the ugly number

 

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next level

class Solution {
    public int nthUglyNumber(int n) {
        // can store the samle element
        //use long to safe
        PriorityQueue<Long> pq = new PriorityQueue<>();
        pq.offer(1L);
        int i = 2;
        while(i<=n){
            long smallest = pq.poll();
            while(!pq.isEmpty() && smallest == pq.peek()) pq.poll();
            pq.offer(smallest*2);
            pq.offer(smallest*3);
            pq.offer(smallest*5);
            i++;
        }
        
        return pq.poll().intValue();
    }
}

Key point: use the feature of PriorityQueue: take the min heap.

• PQ could have duplictae number
• Integer is not enough for this problem (negative will affect the PQ)

 

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next level there are some methods

class Solution {
    public int nthSuperUglyNumber(int n, int[] primes) {
         // can store the samle element
        //use long to safe
        PriorityQueue<Long> pq = new PriorityQueue<>();
        pq.offer(1L);
        int i = 2;
        while(i<=n){
            long smallest = pq.poll();
            while(!pq.isEmpty() && smallest == pq.peek()) pq.poll();
            for(int j = 0; j<primes.length; j++){
                pq.offer(smallest * primes[j]);
            }
            i++;
        }
        
        return pq.poll().intValue();
    }
}

https://leetcode.com/problems/super-ugly-number/discuss/76291/Java-three-methods-23ms-36-ms-58ms(with-heap)-performance-explained -- mark to learn