[Luogu 3128] USACO15DEC Max Flow

[Luogu 3128] USACO15DEC Max Flow

<题目链接>

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最近跟 LCA 干上了…

树剖好啊，我再也不想写倍增了。

以及似乎成功转成了空格选手 qwq。

对于每两个点 S and T，求一下 LCA 顺便树上差分，最后求差分数组的前缀和并找出最大值输出就行了。

（PS：最近考前训练不开 C++11，所以如果看见我写了奇怪的 define 请自动无视QAQ！）

#include <algorithm>
#include <cstdio>

#define nullptr NULL

const int MAXN = 50010; 

int n, m;

namespace HLD
{
    int num, qwq[MAXN];
    struct Node
    {
        int depth, father, son, top, size, DFN;
    }s[MAXN];
    class Graph
    {
        private:
            struct Edge
            {
                int to;
                Edge *next;
                Edge(int to, Edge* next): to(to), next(next) {}
                ~Edge(void)
                {
                    if(next != nullptr)
                        delete next;
                }
            }*head[MAXN];
            void AddEdges(int u, int v)
            {
                head[u] = new Edge(v, head[u]);
                head[v] = new Edge(u, head[v]);
            }
            void DFS1(int u, int k)
            {
                s[u].depth = k;
                s[u].size = 1;
                int v;
                for(Edge *i = head[u]; i != nullptr; i = i -> next)
                    if(!s[v = i -> to].size)
                    {
                        DFS1(v, k + 1);
                        s[u].size += s[v].size;
                        s[v].father = u;
                        if(s[v].size > s[s[u].son].size)
                            s[u].son = v;
                    }
            }
            void DFS2(int u, int top)
            {
                s[u].top = top;
                s[u].DFN = ++num;
                if(s[u].son)
                    DFS2(s[u].son, top);
                int v;
                for(Edge *i = head[u]; i != nullptr; i = i -> next)
                    if(!s[v = i -> to].top)
                        DFS2(v, v);
            }
            void Modify(int x, int y)
            {
                ++qwq[s[x].DFN];
                --qwq[s[y].DFN + 1];
            }
        public:
            Graph(int n)
            {
                std :: fill(head + 1, head + n + 1, (Edge*)nullptr);
                for(int i = 1, x, y; i < n; ++i)
                {
                    scanf("%d %d", &x, &y);
                    AddEdges(x, y);
                }
                DFS1(1, 1);
                DFS2(1, 1);
            }
            ~Graph(void)
            {
                for(int i = 1; i <= n; ++i)
                    delete head[i];
            }
            void Run(int x, int y)
            {
                int a, b;
                while((a = s[x].top) ^ (b = s[y].top))
                    if(s[a].depth > s[b].depth)
                    {
                        Modify(a, x);
                        x = s[a].father;
                    }
                    else
                    {
                        Modify(b, y);
                        y = s[b].father;
                    }
                if(s[x].depth < s[y].depth)
                    Modify(x, y);
                else
                    Modify(y, x);
            }
            int Answer(void)
            {
                int ans = qwq[1];
                for(int i = 2; i <= n; ++i)
                    ans = std :: max(ans, qwq[i] += qwq[i-1]);
                return ans;
            }
    }*G;
}

int main(void)
{
    scanf("%d %d", &n, &m);
    HLD :: G = new HLD :: Graph(n);
    for(int i = 1, x, y; i <= m; ++i)
    {
        scanf("%d %d", &x, &y);
        HLD :: G -> Run(x, y);
    }
    printf("%d\n", HLD :: G -> Answer());
    return 0;
}

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谢谢阅读。