Power of Four

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342.Power of Four

Given an integer n, return true if it is a power of four. Otherwise, return false.

An integer n is a power of four, if there exists an integer x such that n == 4x.

Example 1:

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Input: n = 16
Output: true

Example 2:

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Input: n = 5
Output: false

Example 3:

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Input: n = 1
Output: true

时间复杂度：O(logn)

空间复杂度：O(1)

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class Solution {
public:
    bool isPowerOfFour(int n) {
        while(n && n % 4 == 0) {
            n /= 4;
        }
        return n == 1;
    }
};

二进制中1的位置：

时间复杂度：O(1)

空间复杂度：O(1)

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class Solution {
public:
    bool isPowerOfFour(int n) {
        return n > 0 && (n & (n - 1)) == 0 && (n & 0xaaaaaaaa) == 0;
    }
};

取模性质：

时间复杂度：O(1)

空间复杂度：O(1)

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