Reverse Bits in a Number Using Python

Question

This question relates to the leetcode question to reverse bits of a 32 bits unsigned integer.

I tried the following code:

class Solution:
    # @param n, an integer
    # @return an integer
    def reverseBits(self, n):
    
        result = 0
        BITMASK = 1

        for i in range(32):

            temp = n & BITMASK

            result = result | temp

            n = n >> 1
            
            result = result << 1

        return result

The strategy I was trying is to bitwise AND the last bit of the number with 1, then bitwise OR it with the result. The number is then right shifted by 1 to drop last bit, and the result is left shifted to make room for the next bit result. The code doesn't generate the correct result. Can someone help me understand how to fix this code ? Thanks.

return int(bin(n)[2:].zfill(32)[::-1], base=2) ? Very navie approach though — Epsi95
– Epsi95, Commented Feb 21, 2021 at 9:01

Ayan Chawla · Accepted Answer · 2021-02-21 09:43:06Z

3

Try this

class Solution:
    def reverseBits(self, n: int) -> int:
        bitmask = 1
        result = 0
        
        for i in range(31):
            temp = n & bitmask
            result = result | temp
            n = n>>1
            result = result<<1

        temp = n & bitmask
        result = result | temp
        return result

The main thing is that, when your code runs 32 times then at last it again shifts the result which gives the error, we need to run the code 31 times and last step of AND and OR keep it outside of the loop.

answered Feb 21, 2021 at 9:43

Ayan Chawla

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