[洛谷P3376]【模板】网络最大流（ISAP）

C++ Code：（ISAP）

#include <cstdio>
#include <cstring>
#define maxn 1210
#define maxm 120010
int n, m, st, ed;
const int inf = 0x7fffffff;
inline int min(int a, int b) {return a < b ? a : b;}
namespace Network_Flow {
    int st, ed, MF, n;
    int head[maxn], cnt = 2;
    struct Edge {
        int to, nxt, w;
    } e[maxm << 1];
    inline void addE(int a, int b, int c) {
        e[cnt] = (Edge) {b, head[a], c}; head[a] = cnt;
        e[cnt ^ 1] = (Edge) {a, head[b], 0}; head[b] = cnt ^ 1;
        cnt += 2;
    }
    int GAP[maxn], d[maxn];
    int last[maxn];
    int q[maxn], h, t;
    inline void init() {
        GAP[d[ed] = 1] = 1;
        for (int i = 1; i <= n; i++) last[i] = head[i];
        q[h = t = 0] = ed;
        while (h <= t) {
            int u = q[h++];
            for (int i = head[u]; i; i = e[i].nxt) {
                int v = e[i].to;
                if (!d[v]) {
                    d[v] = d[u] + 1;
                    GAP[d[v]]++;
                    q[++t] = v;
                }
            }
        }
    }
    int dfs(int u, int low) {
        if (!low || u == ed) return low;
        int w, res = 0;
        for (int &i = last[u]; i; i = e[i].nxt) {
            int v = e[i].to;
            if (d[u] == d[v] + 1) {
                w = dfs(v, min(low, e[i].w));
                res += w, low -= w;
                e[i].w -= w, e[i ^ 1].w += w;
                if (!low) return res;
            }
        }
        if (!(--GAP[d[u]])) d[st] = n + 1;
        ++GAP[++d[u]], last[u] = head[u];
        return res;
    }
    inline void ISAP(int S, int T) {
        st = S, ed = T;
        init();
        while (d[st] <= n) MF += dfs(st, inf);
    }
    inline void clear() {
        memset(head, 0, sizeof head); cnt = 2;
        memset(GAP, 0, sizeof GAP);
        memset(d, 0, sizeof d);
        MF = 0;
    }
}
#define read() R::READ()
#include <cctype>
namespace R {
    int x;
    #ifdef ONLINE_JUGER
    #define M 1 << 25
    char op[M], *ch;
    inline void init() {
        fread(ch = op, 1, M, stdin);
    }
    inline int READ() {
        while (isspace(*ch)) ch++;
        for (x = *ch & 15, ch++; isdigit(*ch); ch++) x = x * 10 + (*ch & 15);
        return x;
    }
    #undef M
    #else
    char ch;
    inline int READ() {
        ch = getchar();
        while (isspace(ch)) ch = getchar();
        for (x = ch & 15, ch = getchar(); isdigit(ch); ch = getchar()) x = x * 10 + (ch & 15);
        return x;
    }
    #endif
}
int main() {
    #ifdef ONLINE_JUGER
    R::init();
    #endif
    n = read(), m = read(), st = read(), ed = read();
    Network_Flow::n = n;
    for (int i = 0, a, b, c; i < m; i++) {
        a = read(), b = read(), c = read();
        Network_Flow::addE(a, b, c);
    }
    Network_Flow::ISAP(st, ed);
    printf("%d\n", Network_Flow::MF);
    return 0;
}