CF1093D Beautiful Graph

思路：

题目倒是没啥好说的，就是注意memset的效率问题。如果循环多次调用memset去初始化一个比较大的数组，那就会很费时间。就是因为这个被hack了。:(

实现：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

const int MOD = 998244353;
const int MAXN = 300005;

vector<int> G[MAXN];
int col[MAXN];
int n, m;
ll c1 = 0, c2 = 0, bin[MAXN];

bool dfs(int v, int c)
{
    col[v] = c;
    if (c == 1) c1++;
    else c2++;
    for (int i = 0; i < G[v].size(); i++)
    {
        if (col[G[v][i]] == c) return false;
        if (col[G[v][i]] == 0 && !dfs(G[v][i], 3 - c)) return false;
    }
    return true;
}

int main()
{
    bin[0] = 1;
    for (int i = 1; i <= 300000; i++) bin[i] = bin[i - 1] * 2 % MOD;
    int t, v, u;
    scanf("%d", &t);
    while (t--)
    {
        scanf("%d%d", &n, &m);
        for (int i = 1; i <= n; i++) { G[i].clear(); col[i] = 0; }
        for (int i = 0; i < m; i++)
        {
            scanf("%d%d", &u, &v);
            G[u].push_back(v);
            G[v].push_back(u);
        }
        bool flg = true;
        ll ans = 1;
        for (int i = 1; i <= n; i++)
        {
            if (col[i]) continue;
            ll tmp = 0; c1 = c2 = 0;
            if (!dfs(i, 1)) { flg = false; break; }
            tmp = (bin[c1] + bin[c2]) % MOD;
            ans = ans * tmp % MOD;
        }
        if (!flg) puts("0");
        else printf("%lld
", ans);
    }
    return 0;
}