【树的DFS】AcWing846.树的重心

AcWing846.树的重心

题解

树的深度优先遍历：可以算出各子树的结点数

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

const int N  = 1e5 + 10;

int ans = 0x3f3f3f3f, n;
bool st[N];
int h[N], ne[2 * N], e[2 * N], idx;

void add(int a, int b)
{
    ne[idx] = h[a];
    h[a] = idx;
    e[idx ++] = b;
}

int dfs(int u)
{
    int res = 0;
    int ma = 0;
    st[u] = true;
    if(h[u] == -1) return 1;
    
    for(int i = h[u]; ~i; i = ne[i])
    if(!st[e[i]])
    {
        int t = dfs(e[i]);
        ma = max(ma, t);
        res += t;
    }
    res += 1; //加上自己
    ma = max(ma, n - res);
    ans = min(ans, ma);
    return res;
}

int main()
{
    memset(h, -1, sizeof h);
    int a, b;
    scanf("%d", &n);
    for(int i = 0; i < n; ++i)
    {
        scanf("%d%d",&a, &b);
        add(a, b);
        add(b, a);
    }
    dfs(1);
    cout << ans << endl;
    return 0;
}