TLE_solve.cpp

#include <vector>
#include <string>
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <unordered_set>
using namespace std;
class Solution {
	public:
		vector<int> grayCode(int n) {
			vector<int> result;
			vector<int> cur;
			int N = 1;
			for (int i = 0; i < n; ++i)
				N <<= 1;
			dfs(result, cur, n, 0, N);
			return result;
		}
	private:
		void dfs(vector<int> &result, vector<int> cur, int n, int x, int N) { 
			cur.push_back(x);
			if (N == 1) {
				result = vector<int>(begin(cur), end(cur));
				return;
			}
			vector<int> next = getPossibleNext(n, x);
			for (auto i : next) {
				if (find(begin(cur), end(cur), i) == end(cur)) {
					dfs(result, cur, n, i, N - 1);
				}
			}
		}
		vector<int> getPossibleNext(int n, int x) {
			int base = 0x1;
			vector<int> result;
			for (int i = 0; i < n; ++i) {
				result.push_back(base ^ x);
				base <<= 1;
			}
			return result;
		}
};
void print(vector<int> v)
{
	cout << "[";
	for_each(begin(v), end(v), [](int i){cout << i << ' ';});
	cout << "]";
	cout << endl;
}
int main(int argc, char **argv)
{
	Solution solution;
	int n;
	while (cin >> n) {
		print(solution.grayCode(n));
	}
	return 0;
}