思路：

題目倒是沒啥好說的，就是注意memset的效率問題。如果迴圈多次呼叫memset去初始化一個比較大的陣列，那就會很費時間。就是因為這個被hack了。:(

實現：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 
 5 const int MOD = 998244353;
 6 const int MAXN = 300005;
 7 
 8 vector<int> G[MAXN];
 9 int col[MAXN];
10 int n, m;
11 ll c1 = 0
 
, c2 = 0, bin[MAXN];
12 
13 bool dfs(int v, int c)
14 {
15     col[v] = c;
16     if (c == 1) c1++;
17     else c2++;
18     for (int i = 0; i < G[v].size(); i++)
19     {
20         if (col[G[v][i]] == c) return false;
21         if (col[G[v][i]] == 0 && !dfs(G[v][i], 3 - c)) return false;
22
 
     }
23     return true;
24 }
25 
26 int main()
27 {
28     bin[0] = 1;
29     for (int i = 1; i <= 300000; i++) bin[i] = bin[i - 1] * 2 % MOD;
30     int t, v, u;
31     scanf("%d", &t);
32     while (t--)
33     {
34         scanf("%d%d", &n, &m);
35         for (int i = 1; i <= n; i++) { G[i].clear(); col[i] = 0
 
; }
36         for (int i = 0; i < m; i++)
37         {
38             scanf("%d%d", &u, &v);
39             G[u].push_back(v);
40             G[v].push_back(u);
41         }
42         bool flg = true;
43         ll ans = 1;
44         for (int i = 1; i <= n; i++)
45         {
46             if (col[i]) continue;
47             ll tmp = 0; c1 = c2 = 0;
48             if (!dfs(i, 1)) { flg = false; break; }
49             tmp = (bin[c1] + bin[c2]) % MOD;
50             ans = ans * tmp % MOD;
51         }
52         if (!flg) puts("0");
53         else printf("%lld\n", ans);
54     }
55     return 0;
56 }