丑数

题目描述

把只包含质因子2、3和5的数称作丑数（Ugly Number）。例如6、8都是丑数，但14不是，因为它包含质因子7。 习惯上我们把1当做是第一个丑数。求按从小到大的顺序的第N个丑数。

C++11(clang++ 3.9)

class Solution {
public:
    int GetUglyNumber_Solution(int index) {
        // input check
        if(0 == index)
            return 0;
        
        vector<int> ugly(index);
        ugly[0] = 1;

        int pointer_two = 0, pointer_three = 0, pointer_five = 0;
        
        for(int i = 1; i < index; i++)
        {
            ugly[i] = min(min(2 * ugly[pointer_two], 3 * ugly[pointer_three]), 
                          5 * ugly[pointer_five]);
            
            if(ugly[i] == 2 * ugly[pointer_two]  ) pointer_two++;
            if(ugly[i] == 3 * ugly[pointer_three]) pointer_three++;
            if(ugly[i] == 5 * ugly[pointer_five] ) pointer_five++;
        }
 
        return ugly[index - 1];
    }
};