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# Reverse bits of a positive integer number in Python

• Difficulty Level : Easy
• Last Updated : 19 Nov, 2020

Given an positive integer and size of bits, reverse all bits of it and return the number with reversed bits.

Examples:

```Input : n = 1, bitSize=32
Output : 2147483648
On a machine with size of
bit as 32. Reverse of 0....001 is
100....0.

Input : n = 2147483648, bitSize=32
Output : 1
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

We can solve this problem quickly in Python. Approach is very simple,

1. Convert integer number into it’s binary representation using bin(num) function.
2. bin() function appends 0b as a prefix in binary representation of number, skip first two characters of binary representation and reverse remaining part of string.
3. As we know in memory any binary representation of a number is filled with leading zeros after last set bit from left that means we need to append bitSize – len(reversedBits) number of zeros after reversing remaining string.
4. Now convert binary representation into integer number using int(string,base) method.

### How int() method wroks ?

int(string,base) method takes a string and base to identify that string is referring to what number system ( binary=2, heaxadecimal=16, octal=8 etc. ) and converts string into decimal number system accordingly. For example ; int(‘1010’,2) = 10.

 `# Function to reverse bits of positive  ``# integer number ``   ` `def` `reverseBits(num,bitSize): ``   ` `     ``# convert number into binary representation ``     ``# output will be like bin(10) = '0b10101' ``     ``binary ``=` `bin``(num) ``   ` `     ``# skip first two characters of binary ``     ``# representation string and reverse ``     ``# remaining string and then append zeros ``     ``# after it. binary[-1:1:-1]  --> start ``     ``# from last character and reverse it until ``     ``# second last character from left ``     ``reverse ``=` `binary[``-``1``:``1``:``-``1``] ``     ``reverse ``=` `reverse ``+` `(bitSize ``-` `len``(reverse))``*``'0'``   ` `     ``# converts reversed binary string into integer ``     ``print` `(``int``(reverse,``2``)) ``   ` `# Driver program ``if` `__name__ ``=``=` `"__main__"``: ``    ``num ``=` `1``    ``bitSize ``=` `32``    ``reverseBits(num,bitSize)`

Output:

```2147483648
```

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