题目: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。
思路:用一个数组,三个指针,index2,index3和index5,给每个元素乘以2,3,5看哪个刚好比当前nextindex位置的元素大,就放进来
class Solution {
public:
int GetUglyNumber_Solution(int index) {
if(index <=0) return 0;
int uglynumber[index];
uglynumber[0]=1;
int nextindex=1,index2=0,index3=0,index5=0;
while(nextindex<index)
{
uglynumber[nextindex]=min3(uglynumber[index2]*2,uglynumber[index3]*3,
uglynumber[index5]*5);
while(uglynumber[index2]*2<=uglynumber[nextindex]) ++index2;
while(uglynumber[index3]*3<=uglynumber[nextindex]) ++index3;
while(uglynumber[index5]*5<=uglynumber[nextindex]) ++index5;
++nextindex;
}return uglynumber[index-1];
}
int min3(int a,int b,int c)
{
int m = a<b?a:b;
return m<c?m:c;
}
};