面试题34 丑数

题目描述

把只包含因子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。

class Solution {
public:
    int ugly;
    
    int min2(int a, int b){
        return a < b ? a : b;
    }
    
    int min(int a, int b, int c){
        return min2(a, min2(b, c));
    }
        
    int GetUglyNumber_Solution(int index) {
        if (index <= 0)
            return 0;
        int *pUglyNumbers = new int[index];
        pUglyNumbers[0] = 1;
        int nextUglyIndex = 1;
        int *pMultiply2 = pUglyNumbers;
        int *pMultiply3 = pUglyNumbers;
        int *pMultiply5 = pUglyNumbers;
        
        while (nextUglyIndex < index){
            int minNum = min(*pMultiply2 * 2, *pMultiply3 * 3, *pMultiply5 * 5);
            pUglyNumbers[nextUglyIndex] = minNum;
            while (*pMultiply2 * 2 <= pUglyNumbers[nextUglyIndex])
                ++pMultiply2;
            while (*pMultiply3 * 3 <= pUglyNumbers[nextUglyIndex])
                ++pMultiply3;
            while (*pMultiply5 * 5 <= pUglyNumbers[nextUglyIndex])
                ++pMultiply5;
            ++nextUglyIndex;
        }
        
        ugly = pUglyNumbers[nextUglyIndex - 1];
        delete[] pUglyNumbers;
        return ugly;
    }
};