Zoj 3820 building fire stations greedy + Tree Diameter

I don't know why it is right, but I can't give a counterexample. During the competition, my teammates proposed to find the diameter of the tree, but I didn't dare to write it. It's a pity.

The specific method is to first find the diameter of the original tree, and then delete an edge in the middle to change it into two Subtrees. The diameter midpoint of the two Subtrees is the answer.

# Include <cstdio> # include <cstring> # include <algorithm> # include <queue> # include <stack> # include <map> # include <set> # include <climits> # include <iostream> # include <string> using namespace STD; # define MP make_pair # define Pb push_backtypedef long ll; typedef unsigned long ull; typedef vector <int> VI; typedef pair <int, int> PII; typedef pair <double, double> PDD; const int INF = int_max/3; const do Uble EPS = 1e-8; const ll linf = 1e17; const double dinf = 1e60; const int maxn = 2e5 + 10; int head [maxn], NXT [maxn <1], U [maxn <1], V [maxn <1], ecnt; int N, DIST [maxn], pre [maxn], path [maxn], id [maxn]; bool vis [maxn], del [maxn <1]; void Adde (INT Uu, int vv) {u [ecnt] = Uu; V [ecnt] = vv; NXT [ecnt] = head [UU]; head [UU] = ecnt ++;} // The BFS parameter is the starting point, returns the end point (the farthest point from the start point) int BFS (int str, int col) {memset (VIS, 0, sizeof (VIS); vis [STR] = true; Dist [STR] = 0; Pre [STR] =-1; queue <int> q; q. push (STR); int now = 0; while (! Q. Empty () {now = Q. Front (); q. Pop (); Id [now] = Col; For (INT I = head [now]; ~ I; I = NXT [I]) if (! Del [I]) {If (! Vis [V [I]) {pre [V [I] = now; Dist [V [I] = DIST [now] + 1; q. push (V [I]); vis [V [I] = true; }}return now;} void solve () {// The First Time, the diameter of the tree is int S, T, Len = 0, Du, DV; S = BFS (1,-1); t = BFS (S,-1 ); for (INT I = T ;~ I; I = pre [I]) {path [Len ++] = I;} // Delete the center edge for (INT I = T, J = 0 ;~ I; I = pre [I], J ++) {If (J + 1) = Len/2) {du = I; DV = pre [I]; break ;}for (INT I = 0; I <ecnt; I ++) {If (U [I] = du & V [I] = DV) | (U [I] = DV & V [I] = DU) {del [I] = true ;}} // calculate the diameter of int maxlen = 0, P [2], PCNT = 0, for (INT I = 1; I <= N; I ++) for the subtree on both sides) if (ID [I] =-1) {S = BFS (I, PCNT); t = BFS (S, PCNT); Len = 0; for (Int J = T ;~ J; j = pre [J]) path [Len ++] = J; For (Int J = T, K = 0 ;~ J; j = pre [J], K ++) if (k + 1) = (LEN + 1)/2) {P [PCNT ++] = J; break;} maxlen = max (maxlen, Len/2);} printf ("% d \ n", maxlen, P [0], P [1]) ;}int main () {int t; scanf ("% d", & T); While (t --) {ecnt = 0; scanf ("% d", & N); memset (Head,-1, sizeof (head); memset (Del, 0, sizeof (DEL )); for (INT I = 0; I <n-1; I ++) {int A, B; scanf ("% d", & A, & B ); ADDE (a, B); Adde (B, A);} solve ();} return 0 ;}

 

Zoj 3820 building fire stations greedy + Tree Diameter