## Tuesday, November 18, 2014

### [LeetCode] Gray Code

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 - 0
01 - 1
11 - 3
10 - 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.

n = 0: 0
n = 1: 0, 1
n = 2: 00, 01, 11, 10  (0, 1, 3, 2)
n = 3: 000, 001, 011, 010, 110, 111, 101, 100 (0, 1, 3, 2, 6, 7, 5, 4)

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15``` ```class Solution { public: vector grayCode(int n) { vector greySeq; if(n<0) return greySeq; greySeq.push_back(0); int inc = 1; for(int i=1; i<=n; i++) { for(int j=greySeq.size()-1; j>=0; j--) greySeq.push_back(greySeq[j]+inc); inc <<= 1; } return greySeq; } }; ```