Submission #3404235

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Source Code Expand

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

#define rep(i, j, n) for(int i=j;i<n;++i)
#define all(i) i.begin(),i.end()
#define rall(i) i.rbegin(),i.rend()
#define INF 1e9
#define Puts(x) puts(x? "Yes":"No")
const int mod = 1e9 + 7;

typedef long long i64;
typedef pair<int, int> pi;

template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vector<T>>;

i64 gcd(int n, int m) {return (m == 0? n : gcd(m, n % m));}
i64 lcm(int n, int m) {return (n / gcd(n, m) * m);}
int dx[] = {0, 1, 0, -1, 1, 1, -1, -1};
int dy[] = {1, 0, -1, 0, 1, -1, 1, -1};

int n, m;
int a, b;
vt<int> dp, dist;
vvt<int> edge;

void bfs() {
  queue<int> que;
  que.push(a);

  dp[a] = 1;
  while(!que.empty()) {
    int p = que.front();
    que.pop();

    for(auto nxt : edge[p]) {
      if(dp[nxt] == 0) {
        que.push(nxt);
        dp[nxt] = dp[p];
        dist[nxt] = dist[p] + 1;
      }
      else if(dp[nxt] && dist[nxt] == dist[p] + 1) {
        (dp[nxt] += dp[p]) %= mod;
      }
    }
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> n >> a >> b >> m;
  dp.resize(n + 1, 0);
  dist.resize(n + 1, INF);
  edge.resize(n + 1);
  rep(i, 0, m) {
    int x, y;
    cin >> x >> y;
    edge[x].push_back(y);
    edge[y].push_back(x);
  }
  
  bfs();
  cout << dp[b] << endl;
}

Submission Info

Judge Result