Given an unsigned integer, reverse all bits of it and return the number with reversed bits.
Input : n = 1 Output : 2147483648 On a machine with size of unsigned bit as 32. Reverse of 0....001 is 100....0. Input : n = 2147483648 Output : 1
In the previous post we had seen two method that solved this problem in O(n) & O(logn ) time. Here we solve this problem in O(1) time using lookup table. It’s hard to reverse all 32 bits (assuming this as size of int) in one go using lookup table (” because it’s infeasible to create lookup table of size 232-1 “). So we break 32 bits into 8 bits of chunks( lookup table of size 28-1 “0-255”).
in lookup tale we will store reverse of every number that are in a range( 0-255)
LookupTable = 0 | binary 00000000 Reverse 00000000
LookupTable = 128 | binary 00000001 reverse 10000000
LookupTable = 64 | binary 00000010 reverse 01000000
LookupTanle = 192 | binary 00000011 reverse 11000000
LookupTable = 32 | binary 00000100 reverse 00100000
and so on… upto lookuptable.
Let’s take an Example How lookup table work.
let number = 12456
in Binary = 00000000000000000011000010101000
Split it into 8 bits chunks : 00000000 | 00000000 | 00110000 | 10101000 in decimal : 0 0 48 168 reverse each chunks using lookup table : Lookuptable[ 0 ] = 0 | in binary 00000000 Lookuptable[48 ] = 12 | in binary 00001100 Lookuptable = 21 | in binary 00010101 Now Binary : 00000000 | 00000000 | 00001100 | 00010101 Binary chunks after rearrangement : 00010101 | 00001100 | 00000000 | 00000000 Reverse of 12456 is 353107968
Time complexity :O(1)
- Count set bits in an integer using Lookup Table
- Count trailing zero bits using lookup table
- Reverse actual bits of the given number
- Write an Efficient C Program to Reverse Bits of a Number
- Check if bits of a number has count of consecutive set bits in increasing order
- Check if all bits can be made same by flipping two consecutive bits
- Toggle bits of a number except first and last bits
- Print numbers having first and last bits as the only set bits
- Compute the parity of a number using XOR and table look-up
- First and Last Three Bits
- Unset the last m bits
- Sum of numbers with exactly 2 bits set
- Toggle the last m bits
- Set all even bits of a number
- Set all odd bits of a number
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