Codeforces734E【缩点+树的直径】

思路：
想不到树的直径就GG了啊…
先缩点，然后缩成一棵相邻两层节点就是不同的个数，然后如果考虑一棵树是

1-0-1-0-1-0-1-0-1-0

这样的话其实就是中间开始搞，然后最少变node_num/2次变成同一种颜色。

然后考虑一棵复杂的树，当树上最长距离像这样在变的时候其余枝干肯定是会被连带到的！

//#pragma comment(linker, "/STACK:102400000,102400000")#include <stdio.h>#include <string.h>#include <iostream>#include <algorithm>#include <vector>#include <queue>#include <stack>#include <set>#include <map>#include <string>#include <math.h>#include <stdlib.h>#include <time.h>using namespace std;typedef long long LL;#define mem(a, b) memset(a, b, sizeof(a))const int Maxn = 2e5 + 10;struct Edge{    int v, nex;}edge[Maxn<<1], e[Maxn<<1];int head[Maxn], tol, op[Maxn], tal;int val[Maxn], tot, col[Maxn], be[Maxn];int n;void init(){    tol = tot = tal = 0;    memset(op, -1, sizeof(op));    memset(head, -1, sizeof(head));}void add(int u, int v){    edge[tol] = (Edge){v, head[u]}, head[u] = tol++;    edge[tol] = (Edge){u, head[v]}, head[v] = tol++;}void ADD(int u, int v){    e[tal] = (Edge){v, op[u]}, op[u] = tal++;    e[tal] = (Edge){u, op[v]}, op[v] = tal++;}void DFS(int u, int fa){    int v;    if(fa != -1){        if(col[be[fa]] != val[u]){            be[u] = ++tot;            col[be[u]] = val[u];            ADD(be[fa], tot);        }        else be[u] = be[fa];    }    for(int i=head[u]; ~i; i = edge[i].nex){        v = edge[i].v;        if(v == fa) continue;        DFS(v, u);    }}int Mmmax(int x, int y, int z){    x = x > y ? x : y;    return x > z ? x : z;}int dis[Maxn];int ans;void DFS_LEN(int u, int fa, int pre_num){    int v;    int temp, Max1, Max2;    Max1 = Max2 = 0;    dis[u] = 1;    for(int i=op[u]; ~i; i=e[i].nex){        v = e[i].v;        if(v == fa) continue;        temp = Mmmax(Max1, Max2, pre_num);        DFS_LEN(v, u, temp);        if(dis[v] > Max1){            Max2 = Max1;            Max1 = dis[v];        }        else if(dis[v] > Max2) Max2 = dis[v];    }    dis[u] = dis[u] + Max1;    ans = Mmmax(ans, Max1+Max2, Max1+pre_num);}int main(){    int u, v;    scanf("%d", &n);    for(int i=1;i<=n;i++) scanf("%d", &val[i]);    init();    for(int i=1;i<n;i++){        scanf("%d%d", &u, &v);        add(u, v);    }    col[++tot] = val[1];    be[1] = tot;    DFS(1, -1);    ans = 0;    DFS_LEN(1, -1, 0);    printf("%d\n", (ans+1)/2);    return 0;}