LeetCode Top Interview Questions 326. Power of Three (Java版; Easy)

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LeetCode Top Interview Questions 326. Power of Three (Java版; Easy)

题目描述

Given an integer, write a function to determine if it is a power of three.

Example 1:

Input: 27
Output: true
Example 2:

Input: 0
Output: false
Example 3:

Input: 9
Output: true
Example 4:

Input: 45
Output: false
Follow up:
Could you do it without using any loop / recursion?

第一次做; 数学分析:3^19的因数只有20个, 分别是3^0,…3^19

class Solution {
    public boolean isPowerOfThree(int n) {
        // 1162261467 is 3^19,  3^20 is bigger than int 
        return (n>0 && 1162261467%n==0)
    }
}

第一次做;特殊情况:0不是3的幂, 1是3的幂; 三的幂的因数只有三; %3==0的数不一定是3的幂, 比如15, 45; 但3的幂%3==0

class Solution {
    public boolean isPowerOfThree(int n) {
        if(n==0)
            return false;
        if(n==1)
            return true;
        if(n%3==0){
            if(n==3)
                return true;
            else
                return isPowerOfThree(n/3);
        }
        return false;
    }
}

​​力扣官方题解​​; 3^19的因数只有3^0, 3^1, 3^2,…3^19

​​力扣官方题解​​; 比我的递归逻辑清晰

public class Solution {
    public boolean isPowerOfThree(int n) {
        if (n < 1) {
            return false;
        }

        while (n % 3 == 0) {
            n /= 3;
        }

        return n == 1;
    }
}