Submission #63035044

Source Code Expand

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#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 1;
int n, x, y, cnt, head[N], f[N], ans;
struct edge {
    int to, next;
} a[N << 1];
void add(int x, int y)
{
    a[++cnt] = {y, head[x]}, head[x] = cnt;
}
bool cmp(int x, int y)
{
    return x > y;
}
void dfs(int x, int fr)
 
 
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#include <bits/stdc++.h>
using namespace std;

const int N = 2e5 + 1;
int n, x, y, cnt, head[N], f[N], ans;

struct edge {
    int to, next;
} a[N << 1];

void add(int x, int y)
{
    a[++cnt] = {y, head[x]}, head[x] = cnt;
}

bool cmp(int x, int y)
{
    return x > y;
}

void dfs(int x, int fr)
{
    vector<int> v;
    for (int i = head[x]; i; i = a[i].next)
    {
        if (a[i].to ^ fr)
        {
            dfs(a[i].to, x);
            v.push_back(f[a[i].to]);
        }
    }
    sort(v.begin(), v.end(), cmp);
    f[x] = 1;
    
    if (v.size() >= 4)
    {
        ans = max(ans, v[0] + v[1] + v[2] + v[3] + 1);
    }

    if (v.size() >= 3 && fr)
    {
        f[x] = v[0] + v[1] + v[2] + 1;
        ans = max(f[x] + 1, ans);
    }
}

signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);

    cin >> n;
    for (int i = 1; i < n; i++) cin >> x >> y, add(x, y), add(y, x);
    dfs(1, 0);
    cout << (ans == 0 ? -1 : ans);

    return 0;
}

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