题目链接:https://atcoder.jp/contests/abc126/tasks/abc126_f
题目大意
给定两个整数 M 和 K ,用小于 2M 的的所有自然数,每个两个,用这些数排成一个长度为 2M+1 的序列,使得序列满足以下条件:
- 每个自然数只能用 2 次。
- 设序列为 a,$forall_{i < j} 满足a[i] == a[j]$有 a[i] xor a[i + 1] xor……xor a[j] == K。
问这个序列是否存在,存在则输出任意一个,不存在输出 -1。
分析
找规律。
貌似很难,其实巨简单。
首先,如果 K >= 2M ,那就不用考虑了,绝对异或不出来。
其次,当 M == 1 时,K 为 0 则可行,K 为 1 则不可行。
最后是 M > 1 的情况,这里举个 M = 3,K = 4 的例子,某一绝对正确的摆法如下:0,1,2,3,5,6,7,4,7,6,5,3,2,1,0,4,加粗的是 K,说服一下自己,在 M > 1 的时候都能这么摆(实际上确实能这么摆)。
代码如下
1 #include <bits/stdc++.h> 2 using namespace std; 3 4 #define INIT() ios::sync_with_stdio(false);cin.tie(0);cout.tie(0); 5 #define Rep(i,n) for (int i = 0; i < (n); ++i) 6 #define For(i,s,t) for (int i = (s); i <= (t); ++i) 7 #define rFor(i,t,s) for (int i = (t); i >= (s); --i) 8 #define ForLL(i, s, t) for (LL i = LL(s); i <= LL(t); ++i) 9 #define rForLL(i, t, s) for (LL i = LL(t); i >= LL(s); --i) 10 #define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i) 11 #define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i) 12 13 #define pr(x) cout << #x << " = " << x << " " 14 #define prln(x) cout << #x << " = " << x << endl 15 16 #define LOWBIT(x) ((x)&(-x)) 17 18 #define ALL(x) x.begin(),x.end() 19 #define INS(x) inserter(x,x.begin()) 20 21 #define ms0(a) memset(a,0,sizeof(a)) 22 #define msI(a) memset(a,inf,sizeof(a)) 23 #define msM(a) memset(a,-1,sizeof(a)) 24 25 #define MP make_pair 26 #define PB push_back 27 #define ft first 28 #define sd second 29 30 template<typename T1, typename T2> 31 istream &operator>>(istream &in, pair<T1, T2> &p) { 32 in >> p.first >> p.second; 33 return in; 34 } 35 36 template<typename T> 37 istream &operator>>(istream &in, vector<T> &v) { 38 for (auto &x: v) 39 in >> x; 40 return in; 41 } 42 43 template<typename T1, typename T2> 44 ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) { 45 out << "[" << p.first << ", " << p.second << "]" << " "; 46 return out; 47 } 48 49 inline int gc(){ 50 static const int BUF = 1e7; 51 static char buf[BUF], *bg = buf + BUF, *ed = bg; 52 53 if(bg == ed) fread(bg = buf, 1, BUF, stdin); 54 return *bg++; 55 } 56 57 inline int ri(){ 58 int x = 0, f = 1, c = gc(); 59 for(; c<48||c>57; f = c=='-'?-1:f, c=gc()); 60 for(; c>47&&c<58; x = x*10 + c - 48, c=gc()); 61 return x*f; 62 } 63 64 typedef long long LL; 65 typedef unsigned long long uLL; 66 typedef pair< double, double > PDD; 67 typedef pair< int, int > PII; 68 typedef pair< string, int > PSI; 69 typedef set< int > SI; 70 typedef vector< int > VI; 71 typedef map< int, int > MII; 72 typedef pair< LL, LL > PLL; 73 typedef vector< LL > VL; 74 typedef vector< VL > VVL; 75 const double EPS = 1e-10; 76 const LL inf = 0x7fffffff; 77 const LL infLL = 0x7fffffffffffffffLL; 78 const LL mod = 1e9 + 7; 79 const int maxN = 3e5 + 7; 80 const LL ONE = 1; 81 const LL evenBits = 0xaaaaaaaaaaaaaaaa; 82 const LL oddBits = 0x5555555555555555; 83 84 LL N, M, K; 85 86 int main(){ 87 INIT(); 88 cin >> M >> K; 89 N = (1 << M) - 1; 90 91 if(K > N) cout << -1 << endl; 92 else if(M == 1) { 93 if(K == 1) cout << -1 << endl; 94 else cout << "0 0 1 1" << endl; 95 } 96 else { 97 For(i, 0, N) if(i != K) cout << i << " "; 98 cout << K << " "; 99 rFor(i, N, 0) if(i != K) cout << i << " "; 100 cout << K << " "; 101 cout << endl; 102 } 103 return 0; 104 }