c++ - reverse a number's bits -

here c++ class revering bits leetcode discuss. https://leetcode.com/discuss/29324/c-solution-9ms-without-loop-without-calculation example, given input 43261596 (represented in binary 00000010100101000001111010011100), return 964176192 (represented in binary 00111001011110000010100101000000).

is there can explain it? thank much!!

class solution { public:     uint32_t reversebits(uint32_t n) {         struct bs         {             unsigned int _00:1; unsigned int _01:1; unsigned int _02:1; unsigned int _03:1;             unsigned int _04:1; unsigned int _05:1; unsigned int _06:1; unsigned int _07:1;             unsigned int _08:1; unsigned int _09:1; unsigned int _10:1; unsigned int _11:1;             unsigned int _12:1; unsigned int _13:1; unsigned int _14:1; unsigned int _15:1;             unsigned int _16:1; unsigned int _17:1; unsigned int _18:1; unsigned int _19:1;             unsigned int _20:1; unsigned int _21:1; unsigned int _22:1; unsigned int _23:1;             unsigned int _24:1; unsigned int _25:1; unsigned int _26:1; unsigned int _27:1;             unsigned int _28:1; unsigned int _29:1; unsigned int _30:1; unsigned int _31:1;         } *b = (bs*)&n,          c =          {               b->_31, b->_30, b->_29, b->_28             , b->_27, b->_26, b->_25, b->_24             , b->_23, b->_22, b->_21, b->_20             , b->_19, b->_18, b->_17, b->_16             , b->_15, b->_14, b->_13, b->_12             , b->_11, b->_10, b->_09, b->_08             , b->_07, b->_06, b->_05, b->_04             , b->_03, b->_02, b->_01, b->_00         };          return *(unsigned int *)&c;     } }; 

consider casting providing different layout stencil on memory.

using stencil picture, code layout of stencil of 32-bits on unsigned integer memory location.

so instead of treating memory uint32_t, treating memory 32 bits.

a pointer 32-bit structure created.

the pointer assigned same memory location uint32_t variable.

the pointer allow different treatment of memory location.

a temporary variable, of 32-bits (using structure), created. variable initialized using initialization list.
bit fields in initialization list original variable, listed in reverse order.

so, in list:

  new bit 0 <-- old bit 31     new bit 1 <-- old bit 30   

the foundation of approach relies on initialization lists. author letting compiler reverse bits.