中文题
解题思路:强连通分量模版,裸题
#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;
#define N 10010
#define M 100010
#define min(a, b) ((a) < (b) ? (a): (b))
struct Edge{
int to, next;
}E[M];
int head[N], sccno[N], pre[N], lowlink[N], stack[N];
int tot, dfs_clock, scc_cnt, n, m, top;
void AddEdge(int from, int to) {
E[tot].to = to;
E[tot].next = head[from];
head[from] = tot++;
}
void init() {
memset(head, -1, sizeof(head));
tot = 0;
int u, v;
for (int i = 0; i < m; i++) {
scanf("%d%d", &u, &v);
AddEdge(u, v);
}
}
void dfs(int u) {
pre[u] = lowlink[u] = ++dfs_clock;
stack[++top] = u;
for (int i = head[u]; i != -1; i = E[i].next) {
int v = E[i].to;
if (!pre[v]) {
dfs(v);
lowlink[u] = min(lowlink[u], lowlink[v]);
}
else if (!sccno[v]) {
lowlink[u] = min(lowlink[u], pre[v]);
}
}
if (pre[u] == lowlink[u]) {
scc_cnt++;
while (1) {
int x = stack[top--];
sccno[x] = scc_cnt;
if (x == u)
break;
}
}
}
void solve() {
memset(sccno, 0, sizeof(sccno));
memset(pre, 0, sizeof(pre));
dfs_clock = top = scc_cnt = 0;
dfs(1);
for (int i = 1; i <= n; i++)
if (sccno[i] != 1) {
printf("No\n");
return ;
}
printf("Yes\n");
}
int main() {
while (scanf("%d%d", &n, &m) != EOF && n + m) {
init();
solve();
}
return 0;
}