Question

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

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. 

Example:

Input: n = 10
Output: 12
Explanation: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note:  

1. 1 is typically treated as an ugly number.
2. n does not exceed 1690.

Answer

class Solution {
public:
    int nthUglyNumber(int n) {
        if (n <= 0) return 0;
        vector<int> vec(1, 1);
        int p = 0, q = 0, z = 0;
        int minVal = 1;
        for (int i = 1; i < n; i++) {
            minVal 
 
= min(min(vec[p] * 2, vec[q] * 3), vec[z] * 5);
            vec.push_back(minVal);
            if (vec[p] * 2 == minVal) p++;
            if (vec[q] * 3 == minVal) q++;
            if (vec[z] * 5 == minVal) z++;
        }
        return minVal;
    }
};

　　既然是隻有2,3,5的因子，只需將他們每種情況都考慮了就可以了，排序相當於在min的功能做了。