Input : n = 7, p = 3, b = 1
Output: 15
7 is 00000111 after setting bit at
3rd position it becomes 00001111.

We first create a mask that has set bit only 
at given position using bit wise shift.
      mask = 1 << position

Then to change value of bit to b, we first
make it 0 using below operation
      value & ~mask

After changing it 0, we change it to b by
doing or of above expression with following
(b << p) & mask, i.e., we return
      ((n & ~mask) | (b << p))

Input : n = 7, p = 3, b = 1
Output: 15
7 is 00000111 after setting bit at
3rd position it becomes 00001111.

We first create a mask that has set bit only 
at given position using bit wise shift.
      mask = 1 << position

Then to change value of bit to b, we first
make it 0 using below operation
      value & ~mask

After changing it 0, we change it to b by
doing or of above expression with following
(b << p) & mask, i.e., we return
      ((n & ~mask) | (b << p))