Luogu p3174 [haoi2009] caterpillar tree DP

Portal

Tree-based DP basic question finding the diameter of the weighted tree with vertices

Then, the point weight is the point degree of each point (son [])...

So it can be simplified.

Maintain the maximum and secondary chains starting from the root according to the normal routine

Change + 1 to + son [x]-1 when saving the DP array.

So DP [x] = Maxx + Maxxx + son [x]-1

(Maxx and Maxxx maintain the longest and secondary chains)

While ans get Max

Time cost: 45 min

Code:

 1 #include<cstdio> 2 #include<cstring> 3 #include<algorithm> 4 #include<cmath> 5 #include<queue> 6 #define ms(a,b) memset(a,b,sizeof a) 7 #define rep(i,a,n) for(int i = a;i <= n;i++) 8 #define per(i,n,a) for(int i = n;i >= a;i--) 9 #define inf 214748364710 using namespace std;11 typedef long long ll;12 typedef double D;13 #define eps 1e-814 ll read() {15     ll as = 0,fu = 1;16     char c = getchar();17     while(c < ‘0‘ || c > ‘9‘) {18         if(c == ‘-‘) fu = -1;19         c = getchar();20     }21     while(c >= ‘0‘ && c <= ‘9‘) {22         as = as * 10 + c - ‘0‘;23         c = getchar();24     }25     return as * fu;26 }27 //head28 const int N = 300003;29 int n;30 int head[N],nxt[N<<1],mo[N<<1],cnt;31 int son[N];32 void _add(int x,int y) {33     son[x]++;34     mo[++cnt] = y;35     nxt[cnt] = head[x];36     head[x] = cnt;37 }38 void add(int x,int y) {39     if(x^y)_add(x,y),_add(y,x);40 }41 42 int dp[N],ans;43 void dfs(int x,int f) {44     int maxx = 0,maxxx = 0;45     for(int i = head[x];i;i = nxt[i]) {46         int sn = mo[i];47         if(sn == f) continue;48         dfs(sn,x);49         if(dp[sn] > maxx) maxxx = maxx,maxx = dp[sn];50         else if(dp[sn] > maxxx) maxxx = dp[sn];51         dp[x] = max(dp[x],dp[sn] + son[x] - 1);52     }53     ans = max(ans,maxx + maxxx + son[x] - 1);54 }55 56 int main() {57     n = read(),read();58     rep(i,2,n) add(read(),read());59     rep(i,1,n) dp[i] = 1;60     dfs(1,0),printf("%d\n",ans);61     return 0;62 }

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Luogu p3174 [haoi2009] caterpillar tree DP