1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
|
#include <stdio.h>
#include <string.h>
#include <stack>
#include <algorithm>
using namespace std;
int n,m, head[10005], cnt, timestamp[10005], low[10005], t, sccnum;
bool ins[10005];
stack<int> s;
struct Arc
{
int from, to, nxt;
}g[100005];
void addArc(int a, int b)
{
g[cnt].from = a, g[cnt].to = b, g[cnt].nxt = head[a];
head[a] = cnt++;
}
void dfs(int i)
{
timestamp[i] = low[i] = ++t;
s.push(i), ins[i] = true;
for (int h = head[i],to; ~h; h = g[h].nxt)
{
to = g[h].to;
if (!timestamp[to])
{
dfs(to);
low[i] = min(low[i], low[to]);
}
else if (ins[to])
{
low[i] = min(low[i], low[to]);
}
}
if (low[i] ==timestamp[i])
{
if (++sccnum>1)
{
return;
}
int j = i;
do
{
j = s.top(), s.pop(), ins[j] = false;
} while (j!=i);
}
}
bool tarjan()
{
for (int i = 1; i<=n; i++)
{
if (!timestamp[i])
{
dfs(i);
if (sccnum>1)
{
return true;
}
}
}
return false;
}
int main() {
#ifdef LOCAL
freopen("d:\\data.in", "r", stdin);
#endif
while(scanf("%d%d", &n, &m), n||m)
{
memset(head, -1, sizeof(head));
memset(ins, 0, sizeof(ins));
memset(timestamp, 0, sizeof(timestamp));
memset(low, 0, sizeof(low));
while(!s.empty())
{
s.pop();
}
cnt = t = sccnum = 0;
while(m--)
{
int a,b;
scanf("%d%d", &a, &b);
addArc(a,b);
}
tarjan()?puts("No"):puts("Yes");
}
return 0;
}
|