190. Reverse Bits #

Problem #

Reverse bits of a given 32 bits unsigned integer.

Example 1:

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

Example 2:

Input: 11111111111111111111111111111101
Output: 10111111111111111111111111111111
Explanation: The input binary string 11111111111111111111111111111101 represents the unsigned integer 4294967293, so return 3221225471 which its binary representation is 10101111110010110010011101101001.

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 signed integer type and should not affect your implementation, as the internal binary representation of the integer 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.

Problem Summary #

Reverse the binary bits of a given 32-bit unsigned integer. Notes:

• Please note that in some languages (such as Java), there is no unsigned integer type. In this case, both input and output will be specified as signed integer types and should not affect your implementation, because the internal binary representation of an integer is the same whether it is signed or unsigned.
• In Java, the compiler uses two’s complement notation to represent signed integers. Therefore, in Example 2 above, the input represents the signed integer -3, and the output represents the signed integer -1073741825.

Solution Approach #

• Easy problem: reverse the 32-bit binary bits.
• Shift num to the right, continuously eliminating the lowest 1 bit on the right, give this 1 to res, and continuously shift res to the left to achieve the goal of reversing the binary bits.

Code #

package leetcode

func reverseBits(num uint32) uint32 {
	var res uint32
	for i := 0; i < 32; i++ {
		res = res<<1 | num&1
		num >>= 1
	}
	return res
}