[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.

https://oj.leetcode.com/problems/gray-code/

思路：格雷码的 定义

思路：递归生成码表法。

思路2：公式法。gray = (binary) xor (binary >> 1)

import java.util.ArrayList;
import java.util.List;

public class Solution {

public List<Integer> grayCode(int n) {
List<Integer> res = new ArrayList<Integer>();
if (n == 0) {
res.add(0);
return res;
}
List<Integer> pre = grayCode(n - 1);
res.addAll(pre);
int high = 1 << n - 1;
for (int i = pre.size() - 1; i >= 0; i--)
res.add(high + pre.get(i));

return res;

}

public List<Integer> grayCode2(int n) {
List<Integer> res = new ArrayList<Integer>();
int count = 1 << n;
for (int i = 0; i < count; i++) {
res.add(i ^ (i >> 1));
}
return res;
}

public static void main(String[] args) {
System.out.println(new Solution().grayCode(3));
}

}

View Code

第二遍记录：

学会递归生成法，注意n=0时候终止条件的处理。

参考：

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