leetcode264. Ugly Number II

264. Ugly Number II

Write a program to find the n-th ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

解法一

三个序列，2的倍数，3的倍数，5的倍数，每次取最小的值。然后再乘对应的factor（2，3，5）。

public class Solution {
    public int nthUglyNumber(int n) {
        int[] ugly = new int[n];
        ugly[0] = 1;
        int index2 = 0, index3 = 0, index5 = 0;
        int factor2 = 2, factor3 = 3, factor5 = 5;
        for (int i = 1; i < n; i++) {
            int min = Math.min(Math.min(factor2, factor3), factor5);
            ugly[i] = min;
            if (min == factor2) {
                factor2 = 2 * ugly[++index2];
            }
            if (min == factor3) {
                factor3 = 3 * ugly[++index3];
            }
            if (min == factor5) {
                factor5 = 5 * ugly[++index5];
            }
        }

        return ugly[n - 1]; 
    }
}

解法二

public class Solution {
    public int nthUglyNumber(int n) {
        if(n <= 0) return -1; // get rid of corner cases 
        if(n == 1) return 1; // base case

        int t2 = 0, t3 = 0, t5 = 0; //pointers for 2, 3, 5
        int[] k = new int[n];
        k[0] = 1;
        for(int i  = 1; i < n ; i ++)
        {
            k[i] = Math.min(k[t2] * 2, Math.min(k[t3] * 3,k[t5] * 5));
            if(k[i] == k[t2] * 2) t2++; 
            if(k[i] == k[t3] * 3) t3++;
            if(k[i] == k[t5] * 5) t5++;
        }

        return k[n-1];
    }
}