# Tips！ Judge whether a number is the power of 4?

Time：2021-2-24

Long lost Shuangbai, QAQ
Leetcode-342

The topic said not to use loops or recursion as much as possible, so I thought of the previous judgment that a number is not a power of 2. Therefore, let’s see if there is any similarity.

The power of 4 must be the power of 2, but the reverse is not true. For example, 8 is to the third power of 2, but not to the power of 4.

The conversion from 1 to 2 is 1;
4 to binary is 100;
6 to binary is 110;
8 to binary is 1000;
The conversion from 16 to 2 is 10000;

To judge whether a number is the power of 2, we need to see if all digits have only one 1 after the number is converted to binary.
The law of the power of 4 is to see if there is only one 1 in all digits after the number is converted to binary, and the 1 can only appear on odd digits.

code：

``````class Solution {
public:
bool isPowerOfFour(int num) {
if(num == 1) return true;
if(num <= 3) return false;
int count = 0;
int i = 1;
while(num != 0){
if((num&1) != 0 && (i&1) != 0){
count++;
}
if((i&1) == 0 && (num&1) != 0){
return false;
}
num>>=1;
i++;
}
if(count == 1)
return true;
return false;
}
};``````

Compared with circulation and other methods, this kind of solution certainly saves a lot of effort.

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