[LeetCode] 033: Gray Code

[Problem]

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.

[Solution]

class Solution {
public:
    // get the nthBit, index starts from 0
    int nthBit(int num, int n){
        return num & (1 << n);
    }
    // convert integer to gray
    int int2gray(int num, int n){
        for(int i = 0; i < n-1; ++i){
            int XOR = nthBit(num, i) ^ (nthBit(num, i+1)>>1);
            num = num & (~nthBit(num, i)) | XOR;
        }
return num;
    }
    // get gray codes with n bits
    vector<int> grayCode(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> res;
        if(n < 0){
            return res;
        }
    if(n == 0){
        res.push_back(0);
        return res;
    }
    
    // generate
    int m = pow(2, n);
        for(int i = 0; i < m; ++i){
            res.push_back(int2gray(i, n));
        }
        return res;
    }
};

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