lintcode-medium-Ugly Number II

Posted 2020-07-06 哥布林工程师

Ugly number is a number that only have factors 2, 3 and 5.

Design an algorithm to find the nth ugly number. The first 10 ugly numbers are 1, 2, 3, 4, 5, 6, 8, 9, 10, 12...

 

Notice

Note that 1 is typically treated as an ugly number.

Example

If n=9, return 10.

Challenge

O(n log n) or O(n) time.

 

class Solution {
    /**
     * @param n an integer
     * @return the nth prime number as description.
     */
    public int nthUglyNumber(int n) {
        // Write your code here
        
        if(n <= 0)
            return 0;
        
        ArrayList<Integer> res = new ArrayList<Integer>();
        res.add(1);
        
        int cur = 1;
        int p1 = 0;
        int p2 = 0;
        int p3 = 0;
        
        int min1 = 1;
        int min2 = 1;
        int min3 = 1;
        
        while(res.size() < n){
            
            while(res.get(p1) * 2 <= cur)
                p1++;
            min1 = res.get(p1) * 2;
            
            while(res.get(p2) * 3 <= cur)
                p2++;
            min2 = res.get(p2) * 3;
            
            while(res.get(p3) * 5 <= cur)
                p3++;
            min3 = res.get(p3) * 5;
            
            int next = Math.min(min1, Math.min(min2, min3));
            
            res.add(next);
            cur = next;
        }
        
        return res.get(n - 1);
    }
    
};