树上点分治 poj 1741

Give a tree with n vertices,each edge has a length(positive integer less than 1001).
Define dist(u,v)=The min distance between node u and v.

Give an integer k,for every pair (u,v) of vertices is called valid if and only if dist(u,v) not exceed k.

Write a program that will count how many pairs which are valid for a given tree.

Input

The input contains several test cases. The first line of each
test case contains two integers n, k. (n<=10000) The following n-1
lines each contains three integers u,v,l, which means there is an edge
between node u and v of length l.

The last test case is followed by two zeros.

Output

For each test case output the answer on a single line.

Sample Input

5 4
1 2 3
1 3 1
1 4 2
3 5 1
0 0

Sample Output

8题目分析 ： 给定一棵树，以及树上边的关系大小，问你又多少对点的距离是小于等于所给定的 d 的思路分析 ： 树上点分治的板子题，首先寻找树的重心，以重心为根结点，寻求所有符合题意要求的点对，但是这样计算会算出一些不符合题目的点对，在减去即可，此时当遍历到一个新的结点时，此时的情况又可以当成最初的情况，找重心的时候要注意，对它的子树来说，总的结点数是小于 n 的！！！代码示例 ：

const int maxn = 1e4+5;
const int inf = 0x3f3f3f3f;
#define ll long long

int n, m;
struct node
{
    int to, cost;
    node(int _to = 0, int _cost = 0):to(_to), cost(_cost){}
};
vector<node>ve[maxn];
int root;
int size[maxn], mx[maxn]; // size表示每个结点所连的结点数， mx表示对每个根结点所连的最大结点子树有多少的结点
int balance;
bool done[maxn];
int ans = 0;
int numm; // 表示结点总数

void getroot(int x, int fa){
    size[x] = 1, mx[x] = 0;

    for(int i = 0; i < ve[x].size(); i++){
        int to = ve[x][i].to;
        if (to == fa || done[to]) continue;
        getroot(to, x);
        size[x] += size[to];
        mx[x] = max(mx[x], size[to]);
    }
    mx[x] = max(mx[x], numm-size[x]); // 对子树在寻找子树的重心的过程中，子树的总结点数是会变小的
    if (mx[x] < balance) {balance = mx[x], root = x;}
}

int cnt = 0;
int dep[maxn];
void dfssize(int x, int fa, int d){
    dep[cnt++] = d;

    for(int i = 0; i < ve[x].size(); i++){
        int to = ve[x][i].to;
        int cost = ve[x][i].cost;
        if (to == fa || done[to]) continue;
        dfssize(to, x, d+cost);
    }
}

int cal(int x, int d){
    cnt = 0;
    dfssize(x, x, d);
    sort(dep, dep+cnt);
    int l = 0, r = cnt-1;
    int sum = 0;

    while(l < r){
        if (dep[l]+dep[r] <= m){
            sum += r-l;
            l++;
        }
        else r--;
    }
    //printf("sum = %d \n", sum);
    //system("pause");
    return sum;
}

void dfs(int x){
    done[x] = true;
    ans += cal(x, 0);

    for(int i = 0; i < ve[x].size(); i++){
        int to = ve[x][i].to;
        int cost = ve[x][i].cost;

        if (done[to]) continue;
        ans -= cal(to, cost);
        balance = inf;
        numm = size[to]; // 这里是重点，因为这个地方一直T，还以为写的代码有问题
        getroot(to, to);
        //printf("root = %d\n", root);
        dfs(root);
    }
}

int a, b, w;
int main() {
    while(scanf("%d%d", &n, &m) && n+m){
        for(int i = 0; i <= 10000; i++) ve[i].clear();
        memset(done, false, sizeof(done));
        for(int i = 1; i < n; i++){
            scanf("%d%d%d", &a, &b, &w);
            ve[a].push_back(node(b, w));
            ve[b].push_back(node(a, w));
        }
        ans = 0;
        balance = inf;
        numm = n;
        getroot(1, 1);
        //printf("root = %d\n", root);
        dfs(root);
        printf("%d\n", ans);
    }
    return 0;
}

原文地址：https://www.cnblogs.com/ccut-ry/p/8481441.html