Topic
- Math
Description
https://leetcode.com/problems/power-of-three/
Given an integer n, return true if it is a power of three. Otherwise, return false.
An integer n is a power of three, if there exists an integer x such that n == 3x.
Example 1:
Input: n = 27
Output: true
Example 2:
Input: n = 0
Output: false
Example 3:
Input: n = 9
Output: true
Example 4:
Input: n = 45
Output: false
Constraints:
- -2³¹ <= n <= 2³¹ - 1
Analysis
略
Submission
public class PowerOfThree {
//方法一：switch版
public boolean isPowerOfThree1(int num) {
switch (num) {
case 1: return true;
case 3: return true;
case 9: return true;
case 27: return true;
case 81: return true;
case 243: return true;
case 729: return true;
case 2187: return true;
case 6561: return true;
case 19683: return true;
case 59049: return true;
case 177147: return true;
case 531441: return true;
case 1594323: return true;
case 4782969: return true;
case 14348907: return true;
case 43046721: return true;
case 129140163: return true;
case 387420489: return true;
case 1162261467: return true;
default: return false;
}
}
//方法二：求余
public boolean isPowerOfThree2(int n) {
// 1162261467 is 3^19, 3^20 is bigger than int
return n > 0 && 1162261467 % n == 0;
}
//方法三：递归
public boolean isPowerOfThree3(int n) {
return n > 0 && (n == 1 || (n % 3 == 0 && isPowerOfThree3(n / 3)));
}
//方法四：迭代
public boolean isPowerOfThree4(int n) {
if (n > 1)
while (n % 3 == 0)
n /= 3;
return n == 1;
}
private static HashSet<Integer> POWER_OF_THREE_SET = new HashSet<>(
Arrays.asList(1, 3, 9, 27, 81, 243, 729, 2187, 6561, 19683, 59049, 177147, 531441, 1594323, 4782969,
14348907, 43046721, 129140163, 387420489, 1162261467));
//方法五：哈希表缓存
public boolean isPowerOfThree5(int n) {
return POWER_OF_THREE_SET.contains(n);
}
private static int[] POWER_OF_THREE_LIST = new int[] {
1, 3, 9, 27, 81, 243, 729, 2187, 6561, 19683, 59049, 177147,
531441, 1594323, 4782969, 14348907, 43046721, 129140163, 387420489, 1162261467 };
//方法六：二分查找缓存
public boolean isPowerOfThree6(int n) {
return Arrays.binarySearch(POWER_OF_THREE_LIST, n) >= 0;
}
}
Test
import static org.junit.Assert.*;
import java.util.ArrayList;
import java.util.List;
import org.junit.Test;
public class PowerOfThreeTest {
public static List<Integer> POWER_OF_THREE_LIST;
static {
List<Integer> result = new ArrayList<Integer>();
for(int i = 0; ; i++) {
int product = 1, lastPrduct = 0;
for(int j = 1; j <= i; j++) {
lastPrduct = product;
product *= 3;
}
if(product / 3 != lastPrduct)
break;
//System.out.println(product + " : " + Integer.toBinaryString(product));
//System.out.println("case " + product + ": return true;");
result.add(product);
}
POWER_OF_THREE_LIST = result;
}
@Test
public void test() {
PowerOfThree obj = new PowerOfThree();
for(int num : POWER_OF_THREE_LIST) {
assertTrue(obj.isPowerOfThree1(num));
assertTrue(obj.isPowerOfThree2(num));
assertTrue(obj.isPowerOfThree3(num));
assertTrue(obj.isPowerOfThree4(num));
assertTrue(obj.isPowerOfThree5(num));
assertTrue(obj.isPowerOfThree6(num));
}
}
}