CodeAgon Problem 2

#define ll long long int

ll mod = 1000000007;
ll ans = 2;

void bfs(vector<vector<ll>> &adj,int k)
{
    int n = adj.size()-1;
    vector<bool> vis(n+1,false);
    queue<ll> q;
    q.push(1);
    unordered_map<ll,ll> m;
    vis[1] = true;
    while(!q.empty())
    {
        int curr = q.front();
        q.pop();
        for(int x:adj[curr])
        {
            if(vis[x])
                continue;
            m[x] = m[curr] + 1;
            vis[x] = true;
            q.push(x);
            if(m[x]>=k)
                continue;
            ans = (ans*2ll)%mod;
        }
    }
}



int Solution::solve(vector<vector<int> > &edge, int k) {
    int n = edge.size()+1;
    vector<vector<ll>> adj(n+1);
    for(vector<int> v:edge)
    {
        adj[v[0]].push_back(v[1]);
        adj[v[1]].push_back(v[0]);
    }
    bfs(adj,k);
    return ans;
}