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topic:
We call numbers that only contain prime factors 2, 3, and 5 as Ugly Numbers. Find the nth ugly number in ascending order.
Example:
输入: n = 10
输出: 12
解释: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 是前 10 个丑数。
Description:
- 1 is an ugly number.
- n does not exceed 1690.
Note: This question is the same as the main site 264 question: https://leetcode-cn.com/problems/ugly-number-ii/
Solution 1: heap
/**
* 思路：
* 丑数：从1开始，当前的数*235，插入堆中，之后删除堆顶元素加入到result中
* 在用堆顶元素*235,将新的丑数加入到堆中，并用set集合去重。循环这个过程
*/
public int nthUglyNumber(int n) {
PriorityQueue<Long> heap = new PriorityQueue<>();
heap.add(1L);
long[] res = new long[n + 1];
int[] primes={
2,3,5};
HashSet<Long> set = new HashSet<>();
int index=1;
while (!heap.isEmpty()) {
Long remove = heap.remove();
for (int prime : primes) {
long uglyNum = remove * prime;
if (!set.contains(uglyNum)) {
set.add(uglyNum);
heap.add(uglyNum);
}
}
res[index++] = remove;
if (index == res.length) return (int)res[n];
}
return (int)res[n];
}
Time complexity: On
Space complexity: On
