丑数

题目：我们把只包含因子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1500个丑数
暴力求解方法虽然简单，但是时间复杂度太高。剑指offer上有如下代码：

int ThreeMin(int a, int b, int c){
    int result = a<b?a:b;
    result = result<c?result:c;
    return result;
}
int GetUglyNumber(int index){
    if(index <= 0)
        return 0;
    int *pUglyNumbers = new int[index+1];
    pUglyNumbers[0] = 1;
    int *p2 = pUglyNumbers;
    int *p3 = pUglyNumbers;
    int *p5 = pUglyNumbers;
    int nextUglyIndex = 1;
    while(nextUglyIndex < index+1){//之所以是index+1,是因为数组pUglyNumbers第0个位置被填充为1，所以元素都相应后移一位，才可以
        int minNum = ThreeMin(*p2*2, *p3*3, *p5*5);
        pUglyNumbers[nextUglyIndex] = minNum;
        while(*p2*2 <= pUglyNumbers[nextUglyIndex])
            ++p2;
        while(*p3*3 <= pUglyNumbers[nextUglyIndex])
            ++p3;
        while(*p5*5 <= pUglyNumbers[nextUglyIndex])
            ++p5;
        ++nextUglyIndex;        
    }
    int ugly = pUglyNumbers[nextUglyIndex-1];
    delete []pUglyNumbers;
    return ugly;
}