POJ 1985 Cow Marathon (tree-shaped DP, diameter of tree)

Test instructions: Given a tree, then let you find out its diameter, which is the furthest distance in two points.

Analysis: Obviously this is a tree DP, there should be three ways, namely two times DFS, two bfs, and one DFS, I only wrote the latter two.

The code is as follows:

Two times BFS:

#include <iostream> #include <cstdio> #include <cstring> #include <vector> #include <queue >using namespace Std;const int maxn = 1e5 + 5;int d[maxn];bool VIS[MAXN], Vvis[maxn];//vis is the mark of BFS, Vvis is the total Mark Vector<i Nt> G[MAXN], w[maxn];//idea is to first find a point in the tree, and then from this point to find, find the furthest point,//And then two from the furthest point to find the farthest point, the maximum value of all the points is the diameter of the tree//d[i] represents the longest distance to node I int BFS (    int root) {memset (Vis, false, sizeof (VIS));    memset (d, 0, sizeof (d));    Queue<int> Q;    Q.push (root);    int ans = root, m = 0;    Vis[root] = Vvis[root] = true;   while (!q.empty ()) {root = Q.front ();        Q.pop ();            for (int i = 0; i < g[root].size (); ++i) {int u = g[root][i];            if (Vis[u]) continue;            Q.push (U);            Vis[u] = Vvis[u] = true;            D[u] = D[root] + w[root][i];                if (D[u] > m) {ans = u;            m = D[u]; }}} return ans;}        void init (int n) {for (int i = 1; I <= n; ++i) {g[i].clear (); W[i].cLear ();    Vvis[i] = false;    }}int Main () {int n, m, U, V, l;    Char ch;        while (CIN >> n >> m) {init (n);            while (m--) {scanf ("%d%d%d%c", &u, &v, &l, &ch);  G[u].push_back (v);            W[u].push_back (l);  G[v].push_back (U);        W[v].push_back (l);        } int ans = 0;        for (int i = 1; I <= n; ++i) if (!vvis[i]) ans = max (ans, D[bfs (BFS (i))]); two times BFS, the first is to find the farthest point, the second is to find the farthest point of the farthest point    cout << ans << endl; } return 0;}

  DFS at a time:

#include <iostream> #include <cstdio> #include <cstring> #include <vector> #include <queue >using namespace Std;const int maxn = 1e5 + 5;int F[MAXN], G[MAXN], ll[maxn];vector<int> G[MAXN], w[maxn];//idea is mainly    Find the furthest point of a point and add a little farther away is the diameter of the tree int dfs (int root, int fa) {if (F[root]! =-1) return f[root]; if (!    G[root].size ()) return f[root] = 0;    int m = 0, ans = root;        for (int i = 0; i < g[root].size (); ++i) {int u = g[root][i];        if (U = = FA) continue;            if (Dfs (U, root) + w[root][i] > m) {m = F[u] + w[root][i];        ans = u; }} Ll[root] = ans;    int mm = 0;        for (int i = 0; i < g[root].size (); ++i) {int u = g[root][i];        if (U = = FA) continue;    if (F[u] + w[root][i] > mm && u = ll[root]) mm = F[u] + w[root][i];    } G[root] = mm; return f[root] = m;}        void init (int n) {for (int i = 1; I <= n; ++i) {g[i].clear ();        W[i].clear (); F[i] = g[i] = LL[i] =-1;    }}int Main () {int n, m, U, V, l;    Char ch;        while (CIN >> n >> m) {init (n);            while (m--) {scanf ("%d%d%d%c", &u, &v, &l, &ch);  G[u].push_back (v);            W[u].push_back (l);  G[v].push_back (U);        W[v].push_back (l);        } int ans = 0;        for (int i = 1; I <= n; ++i) if (f[i] = = 1) Dfs (I,-1);            for (int i = 1; I <= n; ++i) ans = max (ans, f[i]+g[i]);    cout << ans << endl; } return 0;}

POJ 1985 Cow Marathon (tree-shaped DP, diameter of tree)