leetcode解题: Power of Four (342)

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Given an integer (signed 32 bits), write a function to check whether it is a power of 4.

Example:
Given num = 16, return true. Given num = 5, return false.

Follow up: Could you solve it without loops/recursion?

解法1：log换底

log4(x) == int, 运用log4(x) = log10(x)/log10(4)
C++

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class Solution {
public:
    bool isPowerOfFour(int num) {
        double temp = log10(num) / log10(4);
        return temp == (int) temp;
    }
};

Java

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解法2：power of two + math

如果是4的次方数，那么-1之后一定能被3整除。
要注意的是&的优先级比==小，所以如果写成num & (num - 1) == 0是不对的。
C++

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class Solution {
public:
    bool isPowerOfFour(int num) {
      return num > 0 && !(num & (num - 1)) && (num - 1) % 3 == 0;
    }
};