231. Power of Two.md

231. Power of Two

Question

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

Example 1:

Input: 1
Output: true
Explanation: 20 = 1

Example 2:

Input: 16
Output: true
Explanation: 24 = 16

Example 3:

Input: 218
Output: false

Thinking:

• Method 1：Bit operation

class Solution {
    public boolean isPowerOfTwo(int n) {
        if(n <= 0) return false;
        int a = 1 << 31;
        int count = 0;
        for(int i = 0; i < 32; i++){
            int cur = n & a;
            if(cur != 0) count++;
            a >>>= 1;
        }
        return count == 1;
    }
}

二刷

1. I just checked if a number can be fully divided by 2.

class Solution {
    public boolean isPowerOfTwo(int n) {
        if(n == 1) return true;
        else if(n <= 0) return false;
        while(n != 1){
            if(n % 2 != 0) return false;
            n /= 2;
        }
        return true;
    }
}

2. bit manipulation

class Solution {
    public boolean isPowerOfTwo(int n) {
        if(n <= 0) return false;
        int temp = 0;
        int count = 0;
        for(int i = 0; i < 31; i++){
            temp = n & 1;            
            if(temp == 1){
                count ++;
                if(count > 1) return false;
            }
            n >>= 1;            
        }
        return true;
    }
}