http://acm.hdu.edu.cn/showproblem.php?pid=6241
二分判断答案，可以将原题目的限制转化为子树至多的点up和至少的点down，如果up >= down则说明当前情况可行。
比赛的时候觉得这种做法太简单了给Pass掉了(雾),事实上这种正确性是可以证明的，只要保证对于所有的l <= r以及l1 <= x <= r，则说明x个黑点的情况可行。
#include <map> #include <set> #include <ctime> #include <cmath> #include <queue> #include <stack> #include <vector> #include <string> #include <bitset> #include <cstdio> #include <cstdlib> #include <cstring> #include <sstream> #include <iostream> #include <algorithm> #include <functional> using namespace std; #define For(i, x, y) for(int i=x;i<=y;i++) #define _For(i, x, y) for(int i=x;i>=y;i--) #define Mem(f, x) memset(f,x,sizeof(f)) #define Sca(x) scanf("%d", &x) #define Sca2(x,y) scanf("%d%d",&x,&y) #define Sca3(x,y,z) scanf("%d%d%d",&x,&y,&z) #define Scl(x) scanf("%lld",&x) #define Pri(x) printf("%d\n", x) #define Prl(x) printf("%lld\n",x) #define CLR(u) for(int i=0;i<=N;i++)u[i].clear(); #define LL long long #define ULL unsigned long long #define mp make_pair #define PII pair<int,int> #define PIL pair<int,long long> #define PLL pair<long long,long long> #define pb push_back #define fi first #define se second typedef vector<int> VI; int read(){int x = 0,f = 1;char c = getchar();while (c<'0' || c>'9'){if (c == '-') f = -1;c = getchar();} while (c >= '0'&&c <= '9'){x = x * 10 + c - '0';c = getchar();}return x*f;} const double PI = acos(-1.0); const double eps = 1e-9; const int maxn = 2e5 + 10; const int INF = 0x3f3f3f3f; const int mod = 1e9 + 7; int N,M,K; struct Edge{ int to,next; }edge[maxn * 2]; int head[maxn],tot; void init(){ for(int i = 0 ; i <= N; i ++) head[i] = -1; tot = 0; } void add(int u,int v){ edge[tot].to = v; edge[tot].next = head[u]; head[u] = tot++; } LL l[maxn],r[maxn],up[maxn],down[maxn],sz[maxn]; void dfs(int sum,int u,int la){ LL L = 0 ,R = 0; sz[u] = 1; down[u] = l[u]; up[u] = sum - r[u]; for(int i = head[u]; ~i ; i = edge[i].next){ int v = edge[i].to; if(v == la) continue; dfs(sum,v,u); sz[u] += sz[v]; L += down[v]; R += up[v]; } down[u] = max(down[u],L); up[u] = min(up[u],sz[u]); up[u] = min(up[u],R + 1); } bool check(int x){ dfs(x,1,-1); bool flag = true; if(up[1] < x || down[1] > x) return false; for(int i = 1; i <= N ; i ++){ if(up[i] < down[i]) return false; } return true; } int solve(){ int l = 0,r = N; int ans = -1; while(l <= r){ int m = l + r >> 1; if(check(m)){ r = m - 1; ans = m; }else{ l = m + 1; } } return ans; } int main(){ int T; Sca(T); while(T--){ Sca(N); init(); for(int i = 1; i <= N ; i ++) l[i] = r[i] = 0; for(int i = 1; i <= N - 1; i ++){ int u,v; Sca2(u,v); add(u,v); add(v,u); } Sca(M); while(M--){ int p = read();LL num = read(); l[p] = max(l[p],num); } Sca(M); while(M--){ int p = read();LL num = read(); r[p] = max(r[p],num); } Pri(solve()); } return 0; }