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. **Example 1:** ~~~ Input:?2 Output:?[0,1,3,2] Explanation: 00 - 0 01 - 1 11 - 3 10 - 2 For a given?n, a gray code sequence may not be uniquely defined. For example, [0,2,3,1] is also a valid gray code sequence. 00 - 0 10 - 2 11 - 3 01 - 1 ~~~ **Example 2:** ~~~ Input:?0 Output:?[0] Explanation: We define the gray code sequence to begin with 0. A gray code sequence of n has size = 2n, which for n = 0 the size is 20 = 1. Therefore, for n = 0 the gray code sequence is [0]. ~~~ 比如說我們輸入3,結果是[0,1,3,2,6,7,5,4] 那么總數就是2的三次方=8； 我們把他分成兩瓣，[0,1,3,2]和[6,7,5,4]， 然后！我們把[0,1,3,2]復制一份，然后倒過來[2,3,1,0]。 這個[6,7,5,4]減去[2,3,1,0]等于[4,4,4,4]=2的n-1次方遍歷4次！  ``` /** * @param {number} n * @return {number[]} */ var grayCode = function(n) { var res = [0]; var right; var i = 1; for(;i<=n;i++ ){ right = [...res].reverse().map((item) => item + Math.pow(2, i - 1)); res = res.concat(right); } return res; }; ```