c_cpp 二分检查

Posted 2021-05-14

vector<vi> g;
int V;

bool isBipartiteUtil(int s, vi &color) {
  queue<int> q;
  q.push(s);
  color[s] = 0;
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    for (auto v : g[u]) {

      // if self-edges are possible, and not allowed
      if (u == v)
        return false;

      // if have not assigned a color, assign and push onto queue
      if (color[v] == -1)
        color[v] = 1 - color[u], q.push(v);
      // if color is assigned, and it matches the color of our adjacent parent,
      // not bipartite
      else if (color[v] == color[u])
        return false;
    }
  }
  return true;
}

// Works for graphs that may not be strongly connected
bool isBipartite() {
  vi color(V, -1);
  for (int i = 0; i < V; i++) {
    if (color[i] == -1) {
      if (!isBipartiteUtil(i, color))
        return false;
    }
  }
  return true;
}