luogu 3174

毛毛蟲一個比較簡單的樹形dp
但是因爲我沒有判斷是不是根節點只得了50分
50分代碼

#include <cstdio>
#include <algorithm>
#include <cstring>
#define MAXN 300010

using namespace std;

int n, m, x, y;
int ans, cnt;
int now[MAXN], vis[MAXN];
int ans1[MAXN], maxx[MAXN], maxx1[MAXN], size[MAXN]; // maxx最大值,maxx1次大值

struct E {
    int from, to, nxt;
}es[MAXN * 2];

int read() {
    int f = 1, k = 0;
    char c = getchar();
    while(c > '9' || c < '0') {
        if(c == '-') {
            f = -1;
        }
        c = getchar();
    }
    while(c >= '0' && c <= '9') {
        k = k * 10 + c - '0';
        c = getchar();
    }
    return f * k;
}


void addedge(int a, int b) {
    es[++cnt].from = a;
    es[cnt].to = b;
    es[cnt].nxt = now[a];
    now[a] = cnt;
}

void dfs(int a) {
    vis[a] = 1;
    int sum = 0;
    for(int i = now[a]; i != 0; i = es[i].nxt) {
        if(!vis[es[i].to]) {
            sum ++;
            dfs(es[i].to);
            if(size[es[i].to] >= maxx[a]) {
                maxx1[a] = maxx[a];
                maxx[a] = size[es[i].to];
            } else if(size[es[i].to] >= maxx1[a] && size[es[i].to] < maxx[a]){
                maxx1[a] = size[es[i].to];
            }
        }
    }
    if(!sum) {
        size[a] = 1;
        ans1[a] = 0;
        maxx[a] = 0;
        return ;
    } else {
        size[a] = maxx[a] + sum;
        ans1[a] = size[a] + maxx1[a] - 1;
    }
}
int main() {
    n = read(), m = read();
    for(int i = 1; i <= m; i++) {
        x = read(), y = read();
        addedge(x, y);
        addedge(y, x);
    }
    dfs(1);
    for(int i = 1; i <= n; i ++) {
        ans = max(ans, ans1[i]);
    }
    printf("%d", ans);
    return 0;
}

ac代碼

#include <cstdio>
#include <algorithm>
#include <cstring>
#define MAXN 300010

using namespace std;

int n, m, x, y;
int ans, cnt;
int now[MAXN], vis[MAXN], son[MAXN];
int ans1[MAXN], maxx[MAXN], maxx1[MAXN], size[MAXN]; // maxx最大值,maxx1次大值

struct E {
    int from, to, nxt;
}es[MAXN * 2];

int read() {
    int f = 1, k = 0;
    char c = getchar();
    while(c > '9' || c < '0') {
        if(c == '-') {
            f = -1;
        }
        c = getchar();
    }
    while(c >= '0' && c <= '9') {
        k = k * 10 + c - '0';
        c = getchar();
    }
    return f * k;
}


void addedge(int a, int b) {
    es[++cnt].from = a;
    es[cnt].to = b;
    es[cnt].nxt = now[a];
    now[a] = cnt;
}

void dfs(int a) {
    vis[a] = 1;
    int sum = 0;
    for(int i = now[a]; i != 0; i = es[i].nxt) {
        if(!vis[es[i].to]) {
            sum ++;
            dfs(es[i].to);
            if(size[es[i].to] >= maxx[a]) {
                maxx1[a] = maxx[a];
                maxx[a] = size[es[i].to];
            } else if(size[es[i].to] >= maxx1[a] && size[es[i].to] < maxx[a]){
                maxx1[a] = size[es[i].to];
            }
        }
    }
    if(!sum) {
        size[a] = 1;
        ans1[a] = 0;
        maxx[a] = 0;
        return ;
    } else {
        size[a] = maxx[a] + son[a] - 1;;
        ans1[a] = maxx[a] + maxx1[a] + son[a] - 1;
    }
}

int main() {
    n = read(), m = read();
    for(int i = 1; i <= m; i++) {
        x = read(), y = read();
        addedge(x, y);
        addedge(y, x);
        son[x] ++;
        son[y] ++;
    }
    dfs(1);
    for(int i = 1; i <= n; i ++) {
        ans = max(ans, ans1[i]);
    }
    printf("%d", ans);
    return 0;
}