说一下为什么按LCA深度从深到浅贪心是对的。我们可以直观感受一下，一条的路径会影响以这个lca为根的这颗树中的链，而深度越深，影响范围越小，所以先选影响范围小的路径。

#include 
     
      using namespace std;const int maxn = 100010;int head[maxn], Next[maxn * 2], ver[maxn * 2], tot, f[maxn][20];int deep[maxn], t;struct Edge {	int u, v, lca, lca_deep;	bool operator < (const Edge& rhs) const {		return lca_deep > rhs.lca_deep;	}};Edge a[maxn];bool v[maxn];void add(int x, int y) {	ver[++tot] = y;	Next[tot] = head[x];	head[x] = tot;}queue
      
        q;void bfs() {	deep[1] = 1;	q.push(1);	while(!q.empty()) {		int x = q.front(); 		q.pop();		for (int i = head[x]; i; i = Next[i]) {			int y = ver[i];			if(deep[y]) continue;			deep[y] = deep[x] + 1;			f[y][0] = x;			for (int j = 1; j <= t; j++)				f[y][j] = f[f[y][j - 1]][j - 1];			q.push(y);		}	}}int query(int x, int y) {	if(deep[x] > deep[y]) swap(x, y);	for (int i = t; i >= 0; i--) {		if(deep[f[y][i]] >= deep[x]) y = f[y][i];	}	if(x == y) return x;	for (int i = t; i >= 0; i--)		if(f[x][i] != f[y][i]) {			x = f[x][i], y = f[y][i];		}	return f[x][0];}void dfs(int x, int fa) {	if(v[x]) return;	v[x] = 1;	for (int i = head[x]; i; i = Next[i]) {		int y = ver[i];		if(y == fa) continue;		dfs(y, x);	}}int main() {	int n, m, x, y;	while(~scanf("%d%d", &n, &m)) {		t = (int)(log(n) / log(2)) + 1; 		tot = 0;		for (int i = 1; i <= n; i++) {			head[i] = 0;			v[i] = 0;			for (int j = 0; j <= t; j++)				f[i][j] = 0;			deep[i] = 0;		}//		memset(head, 0, sizeof(head));//		memset(f, 0, sizeof(f));//		memset(v, 0, sizeof(v));//		memset(deep, 0, sizeof(deep));//		tot = 0;		for (int i = 1; i < n; i++) {			scanf("%d%d", &x, &y);			add(x, y);			add(y, x);		}		bfs();		int ans = 0;		for (int i = 1; i <= m; i++) {			scanf("%d%d", &a[i].u, &a[i].v);			a[i].lca = query(a[i].u, a[i].v);			a[i].lca_deep = deep[a[i].lca];		}		sort(a + 1, a + 1 + m);		for (int i = 1; i <= m; i++) {			if(!v[a[i].u] && !v[a[i].v]) {				ans++;				dfs(a[i].lca, f[a[i].lca][0]);			}		}		printf("%d\n", ans);	}}