如果把毛毛虫的毛都剃下去,那么就是求树的直径。现在加上毛,也可以仿照树的直径树形dp的做法。
在dfs的时候开两个变量,分别维护以u为端点的最长链Max1和次长链Max2,然后像求树的直径那样更新即可。
单开一个dp[u]表示在u是的最长链,这个最长链是包含u及其毛的,而上面的不包括,这样方便更新(否则就会像我一样因为多一根毛少一根毛调了半天)。
然后每次用Max1 + Max2 + w[u] - 1更新ans。(w[u]是u的毛,不包含u)
1 #include<cstdio> 2 #include<iostream> 3 #include<cmath> 4 #include<algorithm> 5 #include<cstring> 6 #include<cstdlib> 7 #include<cctype> 8 #include<vector> 9 #include<stack> 10 #include<queue> 11 using namespace std; 12 #define enter puts("") 13 #define space putchar(' ') 14 #define Mem(a, x) memset(a, x, sizeof(a)) 15 #define rg register 16 typedef long long ll; 17 typedef double db; 18 const int INF = 0x3f3f3f3f; 19 const db eps = 1e-8; 20 const int maxn = 3e5 + 5; 21 inline ll read() 22 { 23 ll ans = 0; 24 char ch = getchar(), last = ' '; 25 while(!isdigit(ch)) last = ch, ch = getchar(); 26 while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar(); 27 if(last == '-') ans = -ans; 28 return ans; 29 } 30 inline void write(ll x) 31 { 32 if(x < 0) x = -x, putchar('-'); 33 if(x >= 10) write(x / 10); 34 putchar(x % 10 + '0'); 35 } 36 37 int n, m, w[maxn]; 38 39 struct Edge 40 { 41 int nxt, to; 42 }e[maxn << 1]; 43 int head[maxn], ecnt = -1; 44 void addEdge(int x, int y) 45 { 46 e[++ecnt] = (Edge){head[x], y}; 47 head[x] = ecnt; 48 } 49 50 int dp[maxn], ans = 0; 51 void dfs(int now, int _f) 52 { 53 int Max1 = 0, Max2 = 0; dp[now] = 1; 54 for(int i = head[now], v; i != -1; i = e[i].nxt) 55 { 56 v = e[i].to; 57 if(v == _f) continue; 58 dfs(v, now); 59 if(dp[v] > Max1) Max2 = Max1, Max1 = dp[v]; 60 else if(dp[v] > Max2) Max2 = dp[v]; 61 dp[now] = max(dp[now], dp[v] + w[now] - 1); 62 } 63 ans = max(ans, Max1 + Max2 + w[now] - 1); 64 } 65 66 int main() 67 { 68 Mem(head, -1); 69 n = read(); m = read(); 70 for(int i = 1; i <= m; ++i) 71 { 72 int x = read(), y = read(); 73 w[x]++; w[y]++; 74 addEdge(x, y); addEdge(y, x); 75 } 76 dfs(1, 0); 77 write(ans), enter; 78 return 0; 79 }