The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 001 - 111 - 310 - 2

Note:

For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

//整数n 的格雷码是n ^ (n/2)。

class Solution {public:    vector
     
       grayCode(int n) {        vector
      
        rst;		size_t size = 1 << n;//2^n		rst.reserve(size);		for (size_t i = 0; i < size; i++)			rst.push_back(i ^ (i >> 1));		return  rst;    }};
      
     

vector的reserve()与resize()函数差别:

• vector 的reserve增加了vector的capacity，可是它的size没有改变！

  而resize改变了vector的capacity同一时候也增加了它的size！

• reserve是容器预留空间。但在空间内不真正创建元素对象，所以在没有增加新的对象之前，不能引用容器内的元素。增加新的元素时，要调用push_back()/insert()函数。
• resize是改变容器的大小，且在创建对象，因此，调用这个函数之后。就能够引用容器内的对象了，因此当增加新的元素时，用operator[]操作符，或者用迭代器来引用元素对象。此时再调用push_back()函数。是加在这个新的空间后面的。