12. toggle all bit

 N = 19 = (10011)₂

Bit = 5

Bit = 1<<Bit  = 10000

Bit | (Bit -1)   = 10000 | 01111

                      = 11111

Toggle all bit = N ^ Bit = 10011 ^ 11111

                        = 10011

                        = 11111

                        = 01100 (all bit toggled)

Code : 

#include<bits/stdc++.h> using namespace std; int main() {       int t,n,bit,i,m;     cin>>n;     bit = floor(log2(n)+1);     i = 1<<(bit-1);     i = i | (i-1);     m = n^i;     cout<<m<<endl;   }