洛谷P2740 草地排水

最大流

一道完全符合最大流定义的板子题。。重新学了一次网络流，希望有更深的理解把。。

#include <bits/stdc++.h>
#define INF 0x3f3f3f3f
#define full(a, b) memset(a, b, sizeof a)
using namespace std;
typedef long long ll;
inline int lowbit(int x){ return x & (-x); }
inline int read(){
    int X = 0, w = 0; char ch = 0;
    while(!isdigit(ch)) { w |= ch == '-'; ch = getchar(); }
    while(isdigit(ch)) X = (X << 3) + (X << 1) + (ch ^ 48), ch = getchar();
    return w ? -X : X;
}
inline int gcd(int a, int b){ return a % b ? gcd(b, a % b) : b; }
inline int lcm(int a, int b){ return a / gcd(a, b) * b; }
template<typename T>
inline T max(T x, T y, T z){ return max(max(x, y), z); }
template<typename T>
inline T min(T x, T y, T z){ return min(min(x, y), z); }
template<typename A, typename B, typename C>
inline A fpow(A x, B p, C lyd){
    A ans = 1;
    for(; p; p >>= 1, x = 1LL * x * x % lyd)if(p & 1)ans = 1LL * x * ans % lyd;
    return ans;
}
const int N = 205;
int n, m, cnt, head[N], depth[N];
struct Edge{ int v, next, w; } edge[N<<2];

void addEdge(int a, int b, int w){
    edge[cnt].v = b, edge[cnt].w = w, edge[cnt].next = head[a], head[a] = cnt ++;
    edge[cnt].v = a, edge[cnt].w = 0, edge[cnt].next = head[b], head[b] = cnt ++;
}

bool bfs(){
    full(depth, 0);
    queue<int> q;
    q.push(1);
    depth[1] = 1;
    while(!q.empty()){
        int s = q.front(); q.pop();
        for(int i = head[s]; i != -1; i = edge[i].next){
            int u = edge[i].v;
            if(!depth[u] && edge[i].w > 0){
                depth[u] = depth[s] + 1;
                q.push(u);
            }
        }
    }
    return depth[m] != 0;
}

int dfs(int s, int a){
    if(s == m) return a;
    int flow = 0;
    for(int i = head[s]; i != -1; i = edge[i].next){
        int u = edge[i].v;
        if(depth[u] == depth[s] + 1 && edge[i].w > 0){
            int k = dfs(u, min(a, edge[i].w));
            flow += k, a -= k, edge[i].w -= k, edge[i^1].w += k;
        }
        if(!a) break;
    }
    if(a) depth[s] = -1;
    return flow;
}

int dinic(){
    int ans = 0;
    while(bfs()){
        ans += dfs(1, INF);
    }
    return ans;
}

int main(){

    full(head, -1);
    n = read(), m = read();
    for(int i = 0; i < n; i ++){
        int u = read(), v = read(), w = read();
        addEdge(u, v, w);
    }
    printf("%d\n", dinic());
    return 0;
}