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

. 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.

Hint:

The naive approach is to call

isUgly

for
every number until you reach the nth one. Most numbers are not ugly. Try to focus your effort on generating only the ugly ones.
An ugly number must be multiplied by either 2, 3, or 5 from a smaller ugly number.
The key is how to maintain the order of the ugly numbers. Try a similar approach of merging from three sorted lists: L1,
L2, and L3.

方法一，利用sortedset直接排序。由于set会内部排序，所以代码简单，但是效率不如下面的方法二。

public static int nthUglyNumber(int n) {
SortedSet<Long> set = new TreeSet<Long>();
long ithNum = 1;
set.add(ithNum);
for (int i = 0; i < n; i++) {
ithNum = set.first();
set.add(ithNum * 2);
set.add(ithNum * 3);
set.add(ithNum * 5);
set.remove(ithNum);
}
return (int) ithNum;
}

方法二：按照题目中的提示，维护一个丑数数组：

public static int nthUglyNumber2(int n) {
int l1 = 0, l2 = 0, l3 = 0;
int[] nums = new int
;
nums[0] = 1;
int i = 1;
while (i < n) {
int minNum = Math.min(Math.min(nums[l1] * 2, nums[l2] * 3), nums[l3] * 5);
if (nums[l1] * 2 == minNum) l1++;
if (nums[l2] * 3 == minNum) l2++;
if (nums[l3] * 5 == minNum) l3++;
nums[i++] = minNum;
}
return nums[n - 1];
}