leetcode [264] - Ugly Number II

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.

题目大意：

　　程序输出第n个丑数。（丑数指只能被2，3，5整除的数）

理  解：

　　可以很容易发现，后面的丑数都是在前面的丑数基础上乘以2或者3或者5.

　　设置三个指针分别指向2，3，5对应的"基数"，比较当前三个数的大小，选择最小的数作为当前的丑数。

　　【参考了其他人的想法，指针这个方法真的很奇妙。】

代  码 C++：

　　

class Solution {
public:
    int nthUglyNumber(int n) {
        int *uglyNumber = new int[n];
        uglyNumber[0]=1;
        int nextUglyIndex = 1;
        
        int *uglyNumberOfTwo = uglyNumber;
        int *uglyNumberOfThree = uglyNumber;
        int *uglyNumberOfFive = uglyNumber;
        int min;
        
        while(nextUglyIndex<n){
            min = (((*uglyNumberOfTwo)*2) < ((*uglyNumberOfThree)*3))? ((*uglyNumberOfTwo)*2):((*uglyNumberOfThree)*3);
            min = (min < *uglyNumberOfFive*5)?min:*uglyNumberOfFive*5;
            uglyNumber[nextUglyIndex]=min;
            
            if(*uglyNumberOfTwo*2<=min)
                uglyNumberOfTwo++;
            if(*uglyNumberOfThree*3<=min)
                uglyNumberOfThree++;
            if(*uglyNumberOfFive*5<=min)
                uglyNumberOfFive++;
            
            nextUglyIndex++;
        }
        return uglyNumber[n-1];
    }
};