HDU 4912 Paths on the tree（LCA+贪心）

#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b)	for (int i(a); i <= (b); ++i)
#define dec(i, a, b)	for (int i(a); i >= (b); --i)

const int N = 1e5 + 10;
const int A = 24;

int f[N][A];
int n, m, ans;
int deep[N], c[N];
vector <int> v[N];

struct node{ 
	int x, y, z;
	friend bool operator < (const node &a, const node &b){
		return deep[a.z] > deep[b.z];
	}
} p[N];

void dfs(int x, int fa, int dep){
	deep[x] = dep;
	if (fa){ 
                f[x][0] = fa;
		for (int i = 0; f[f[x][i]][i]; ++i) f[x][i + 1] = f[f[x][i]][i];
	}
	for (auto u : v[x]){
		if (u == fa) continue;
		dfs(u, x, dep + 1);
	}
}

int LCA(int a, int b){
	if (deep[a] < deep[b]) swap(a, b);
	for (int i = 0,  delta = deep[a] - deep[b]; delta; delta >>= 1, ++i) if (delta & 1) a = f[a][i];
	if (a == b) return a;
	dec(i, 19, 0) if (f[a][i] != f[b][i]) a = f[a][i], b = f[b][i];
	return f[a][0];
}

void tag(int x){
	c[x] = 1;
	for (auto u : v[x]){
		if (u == f[x][0]) continue;
		if (c[u] == 0) tag(u);
	}
}

int main(){


	while (~scanf("%d%d", &n, &m)){
		memset(f, 0, sizeof f);
		rep(i, 0, n + 1) v[i].clear();
		rep(i, 2, n){
			int x, y;
			scanf("%d%d", &x, &y);
			v[x].push_back(y);
			v[y].push_back(x);
		}

		memset(deep, 0, sizeof deep);
		dfs(1, 0, 0);
		ans = 0;

		rep(i, 1, m){
			int x, y;
			scanf("%d%d", &x, &y);
			int z = LCA(x, y);
			p[i] = {x, y, z};
		}

		sort(p + 1, p + m + 1);

		memset(c, 0, sizeof c);
		rep(i, 1, m){
			int u = p[i].x, w = p[i].y;
			if (c[u] == 0 && c[w] == 0){
				tag(p[i].z);
				++ans;
			}
		}


		printf("%d\n", ans);


	}
	return 0;
}