「一本通 4.4 例 2」暗的连锁 POJ3417 LOJ10131

树上边的差分，给每个点赋值为0,对于非树边（x,y）x,y权值+1,lca（x,y）-=2,再dfs

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#define MN 200050
using namespace std;
int n, m, cnt, head[MN], ans[MN], dfn[MN], num[MN];
int f[MN][21];
int lg[MN];
struct tu {
    int v, nxt;
} e[MN];
void add(int u, int v) {
    e[++cnt].v = v;
    e[cnt].nxt = head[u];
    head[u] = cnt;
}
void dfs(int now, int fa) {
    dfn[now] = dfn[fa] + 1;
    f[now][0] = fa;
    for (int i = 1; (1 << i) <= dfn[now]; i++) f[now][i] = f[f[now][i - 1]][i - 1];
    for (int i = head[now]; i; i = e[i].nxt) {
        if (e[i].v != fa)
            dfs(e[i].v, now);
    }
}
int lca(int x, int y) {
    if (dfn[x] < dfn[y])
        swap(x, y);
    while (dfn[x] > dfn[y]) {
        x = f[x][lg[dfn[x] - dfn[y]]];
    }
    if (x == y)
        return x;
    for (int k = lg[dfn[x]]; k >= 0; k--)
        if (f[x][k] != f[y][k])
            x = f[x][k], y = f[y][k];
    return f[x][0];
}
void search(int now, int fa) {
    ans[now] = num[now];
    for (int i = head[now]; i; i = e[i].nxt) {
        if (e[i].v != fa) {
            search(e[i].v, now);
            ans[now] += ans[e[i].v];
        }
    }
}
int main() {
    int sum = 0;
    scanf("%d%d", &n, &m);
    lg[0] = -1;
    for (int i = 1; i <= n; i++) lg[i] = lg[i >> 1] + 1;
    for (int i = 1; i <= n - 1; i++) {
        int a, b;
        scanf("%d%d", &a, &b);
        add(a, b);
        add(b, a);
    }
    dfs(1, 0);
    for (int i = 1; i <= m; i++) {
        int a, b;
        scanf("%d%d", &a, &b);
        num[a]++;
        num[b]++;
        num[lca(a, b)] -= 2;
    }
    search(1, 0);
    for (int i = 2; i <= n; i++) {
        if (!ans[i])
            sum += m;
        if (ans[i] == 1)
            sum++;
    }
    printf("%d", sum);

    return 0;
}

 

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