【LeetCode】264. Ugly Number II

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. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note that 1 is typically treated as an ugly number.

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Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

 

思路：全部的ugly number都是由1開始，乘以2/3/5生成的。

只要將這些生成的數排序便可得到，自動排序可使用set

這樣每次取出的第一個元素就是最小元素，由此再繼續生成新的ugly number.

class Solution {
public:
    int nthUglyNumber(int n) {
        set<long long> order;
        order.insert(1);
        int count = 0;
        long long curmin = 0;
        while(count < n)
        {
            curmin = *(order.begin());
            order.erase(order.begin());
            order.insert(curmin * 2);
            order.insert(curmin * 3);
            order.insert(curmin * 5);
            count ++;
        }
        return (int)curmin;
    }
};