190. Reverse Bits

Easy

Reverse bits of a given 32 bits unsigned integer.

Note:

• Note that in some languages, such as Java, there is no unsigned integer type. In this case, both input and output will be given as a signed integer type. They should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned.

• In Java, the compiler represents the signed integers using 2's complement notation. Therefore, in Example 2 above, the input represents the signed integer -3 and the output represents the signed integer -1073741825.

Example 1:

Input: n = 00000010100101000001111010011100
Output:
    964176192 (00111001011110000010100101000000)
Explanation: 
The input binary string 00000010100101000001111010011100 represents the unsigned integer 43261596, so return 964176192 which its binary representation is 00111001011110000010100101000000.

Example 2:

Input: n = 11111111111111111111111111111101
Output:
   3221225471 (10111111111111111111111111111111)
Explanation: 
The input binary string 11111111111111111111111111111101 represents the unsigned integer 4294967293, so return 3221225471 which its binary representation is 10111111111111111111111111111111.

Constraints:

• The input must be a binary string of length 32

Follow up: If this function is called many times, how would you optimize it?

解題

第一種解法是使用 Go bits package 內的函式。

func reverseBits(num uint32) uint32 {
    return bits.Reverse32(num)
}

第二種是透過shift把位元移到reverse後的位置。

func reverseBits(num uint32) uint32 {
    var result uint32   
    for i := 0; i < 32; i++ {
        result += num >> i << 31 >> i
    }
    return result
}