HDU5883 The Best Path(欧拉回路 | 通路下求XOR的最大值)

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <algorithm>
 4 #include <cstring>
 5 #include <cmath>
 6 
 7 using namespace std;
 8 typedef long long LL;
 9 const int MAXN = 100000;
10 const int MAXE = 500000;
11 int a[MAXN + 3], deg[MAXN + 3], pre[MAXN + 3], t, n, m;
14 
15 int Find(int x) { return x == pre[x] ? x : pre[x] = Find(pre[x]); }
16 
17 void mix(int  x, int y) {
18     int fx = Find(x), fy = Find(y);
19     if(fx != fy) pre[fx] = fy;
20 }
21 
22 int isEulr(int n) {
23     int cnt1 = 0, cnt2 = 0;
24     for(int i = 1; i <= n; i++) {
25         if(deg[i] & 1) cnt1++;
26         if(pre[i] == i) cnt2++;
27     }
28     if( (cnt1 == 2 || cnt1 == 0) && cnt2 == 1) return cnt1; //cnt1 为 0 代表欧拉回路, 为 2 代表欧拉通路
29     return -1;
30 }
31 
32 int main(){34     scanf("%d", &t);
35     while(t--) {
36         memset(a, 0, sizeof(a));
37         memset(deg, 0, sizeof(deg));39         scanf("%d%d", &n, &m);
40         for(int i = 1; i <= n; i++) scanf("%d", &a[i]);
41         for(int i = 0; i <= n; i++) pre[i] = i;
42         int u, v, k;
43         for(int i = 0; i < m; i++) {
44             scanf("%d%d", &u, &v);
45             deg[u]++, deg[v]++;
46             mix(u, v);
47         }
48         if( (k = isEulr(n) ) >= 0) {
49             int ans = 0;
50             for(int i = 1; i <= n; i++) ans ^= ( (deg[i] / 2) & 1 ? a[i] : 0 ) ;
51             if(k == 2)for(int i = 1; i <= n; i++)  { if(deg[i] & 1) ans ^= a[i]; }//欧拉通路,起点和终点需要多XOR一次
52             else  for(int i = 1; i <= n; i++)  ans = max(ans, ans ^ a[i]); //欧拉回路, 枚举下起点    
53             printf("%d\n", ans);
54         }
55         else printf("Impossible\n");
56     }
57     return 0;
58 }