Merge pull request #118 from walkerrandolphsmith/feature/89

solve problem

Author: walkerrandolphsmith

leet-code/89-gray-code/README.md

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+# 89-gray-code
+
+An n-bit gray code sequence is a sequence of 2n integers where:
+
+Every integer is in the inclusive range [0, 2n - 1],
+The first integer is 0,
+An integer appears no more than once in the sequence,
+The binary representation of every pair of adjacent integers differs by exactly one bit, and
+The binary representation of the first and last integers differs by exactly one bit.
+Given an integer n, return any valid n-bit gray code sequence.
+
+Example 1:
+
+Input: n = 2
+Output: [0,1,3,2]
+Explanation:
+The binary representation of [0,1,3,2] is [00,01,11,10].
+- 00 and 01 differ by one bit
+- 01 and 11 differ by one bit
+- 11 and 10 differ by one bit
+- 10 and 00 differ by one bit
+[0,2,3,1] is also a valid gray code sequence, whose binary representation is [00,10,11,01].
+- 00 and 10 differ by one bit
+- 10 and 11 differ by one bit
+- 11 and 01 differ by one bit
+- 01 and 00 differ by one bit
+Example 2:
+
+Input: n = 1
+Output: [0,1]
+ 
+
+Constraints:
+
+1 <= n <= 16

leet-code/89-gray-code/index.js

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+/**
+ * @param {number} n
+ * @return {number[]}
+ */
+var grayCode = function(n) {
+    const numberOfBits = 1 << n; // 2^n
+    return Array.from({ length: numberOfBits }, (_, i) => encode(i));
+};
+
+function encode(n) {
+    return xor(n, dropLeastSignificantBit(n))
+}
+
+function dropLeastSignificantBit(n) {
+    return n >> 1
+}
+
+function xor(x, y) {
+    return  x ^ y
+}