Leetcode: 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.

保存丑数序列，维护*2， *3 ，*5的三个队列，采用归并排序的思想选择最小的为下一个丑数。注意有可能某个丑数同时来自不同的队列。

class Solution {
public:
int nthUglyNumber(int n) {
vector<int> nums(n, 1);
int pos2 = 0, pos3 = 0, pos5 = 0;
for (int i = 1; i < n; ++i) {
nums[i] = min(min(nums[pos2] * 2, nums[pos3] * 3), nums[pos5] * 5);
if (nums[i] == nums[pos2] * 2) {
++pos2;
}
if (nums[i] == nums[pos3] * 3) {
++pos3;
}
if (nums[i] == nums[pos5] * 5) {
++pos5;
}
}

return nums[n-1];
}
};