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 is typically treated as an ugly number.
n does not exceed 1690.
难度:medium
题目:写程序找出第n个丑数。丑数公因子只由2,3,5构正的正整数。
思路:三路指针。
Runtime: 2 ms, faster than 99.79% of Java online submissions for Ugly Number II.
Memory Usage: 36.8 MB, less than 32.39% of Java online submissions for Ugly Number II.
class Solution {
public int nthUglyNumber(int n) {
int[] ugly = new int[n];
ugly[0] = 1;
int p2 = 0, p3 = 0, p5 = 0;
for (int i = 1; i < n; i++) {
ugly[i] = Math.min(ugly[p2] * 2, Math.min(ugly[p3] * 3, ugly[p5] * 5));
if (ugly[i] == ugly[p2] * 2) {
p2++;
}
if (ugly[i] == ugly[p3] * 3) {
p3++;
}
if (ugly[i] == ugly[p5] * 5) {
p5++;
}
}
return ugly[n - 1];
}
}
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