注意事项：

  1. 变量 cntnex 初值要设为奇数。

Ek 算法

代码；

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int l, r, que[MAXn + 10];
bool vis[MAXn + 10]; int prec[MAXn + 10], lim[MAXn + 10];
bool bfs(int s, int t) {
    l = 1, r = 0;
    memset(vis, 0, sizeof(vis));
    que[++r] = s;
    vis[s] = 1;
    prec[s] = 0;
    lim[s] = INF;
    while (l <= r) {
        int cur = que[l++];
        for (int i = head[cur]; i; i = nex[i]) {
            if (cap[i] == 0 || vis[to[i]]) continue;
            que[++r] = to[i];
            vis[to[i]] = 1;
            prec[to[i]] = i;
            lim[to[i]] = min(lim[cur], cap[i]);
            if (to[i] == t) return 1;
        }
    }
    return 0;
}

void ek(int s, int t, int &ansf) {
    ansf = 0;
    while (bfs(s, t)) {
        ansf += lim[t];
        int cur = t;
        while (cur != s) {
            cap[prec[cur]] -= lim[t]; cap[prec[cur] ^ 1] += lim[t];
            cur = to[prec[cur] ^ 1];
        }
    }
}

Dinic 算法

代码；

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int arc[MAXn + 10], lay[MAXn + 10];
int l, r, que[MAXn + 10];
bool bfs(int s, int t) {
    l = 1, r = 0;
    memset(lay, -1, sizeof(lay));
    arc[s] = head[s];
    lay[s] = 0;
    que[++r] = s;
    while (l <= r) {
        int cur = que[l++];
        for (int i = head[cur]; i; i = nex[i]) {
            if (cap[i] == 0 || lay[to[i]] != -1) continue;
            arc[to[i]] = head[to[i]];
            lay[to[i]] = lay[cur] + 1;
            if (to[i] == t) return 1;
            que[++r] = to[i];
        }
    }
    return 0;
}
int dfs(int cur, int t, int lim) {
    if (cur == t) return lim;
    int ans = 0;
    for (int i = arc[cur]; i && ans < lim; i = nex[i]) {
        arc[cur] = i;
        if (cap[i] == 0 || lay[to[i]] != lay[cur] + 1) continue;
        int f = dfs(to[i], t, min(lim - ans, cap[i]));
        if (f == 0) lay[to[i]] = -1;
        else {
            ans += f;
            cap[i] -= f; cap[i ^ 1] += f;
        }
    }
    return ans;
}
void dinic(int s, int t, int &ansf) {
    ansf = 0;
    while (bfs(s, t)) ansf += dfs(s, t, INF);
}

题目

Luogu P3376 【模板】网络最大流

代码（以 Dinic 为例）：

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#include<bits/stdc++.h>
using namespace std;
#define int long long
const int MAXn = 2e2;
const int MAXm = 5e3;
const int INF = 0x3f3f3f3f;

template <typename T>
inline void read(T &a) {
    char c;for (c = getchar(); (c < '0' || c > '9') && c != '-'; c = getchar());bool f = c == '-';T x = f ? 0 : (c ^ '0');for (c = getchar(); c >= '0' && c <= '9'; c = getchar()) {x = x * 10 + (c ^ '0');}a = f ? -x : x;
}
template <typename T, typename ...Argv>
inline void read(T &a, Argv &...argv) {
    read(a), read(argv...);
}

int head[MAXn + 10], cntnex = 1, nex[MAXm * 2 + 10], to[MAXm * 2 + 10], cap[MAXm * 2 + 10];
inline void connect(int u, int v, int c) {
    nex[++cntnex] = head[u];
    head[u] = cntnex;
    to[cntnex] = v;
    cap[cntnex] = c;
}

int arc[MAXn + 10], lay[MAXn + 10];
int l, r, que[MAXn + 10];
bool bfs(int s, int t) {
    l = 1, r = 0;
    memset(lay, -1, sizeof(lay));
    arc[s] = head[s];
    lay[s] = 0;
    que[++r] = s;
    while (l <= r) {
        int cur = que[l++];
        for (int i = head[cur]; i; i = nex[i]) {
            if (cap[i] == 0 || lay[to[i]] != -1) continue;
            arc[to[i]] = head[to[i]];
            lay[to[i]] = lay[cur] + 1;
            if (to[i] == t) return 1;
            que[++r] = to[i];
        }
    }
    return 0;
}
int dfs(int cur, int t, int lim) {
    if (cur == t) return lim;
    int ans = 0;
    for (int i = arc[cur]; i && ans < lim; i = nex[i]) {
        arc[cur] = i;
        if (cap[i] == 0 || lay[to[i]] != lay[cur] + 1) continue;
        int f = dfs(to[i], t, min(lim - ans, cap[i]));
        if (f == 0) lay[to[i]] = -1;
        else {
            ans += f;
            cap[i] -= f; cap[i ^ 1] += f;
        }
    }
    return ans;
}
void dinic(int s, int t, int &ansf) {
    ansf = 0;
    while (bfs(s, t)) ansf += dfs(s, t, INF);
}

int n, m, s, t;
signed main() {
    read(n, m, s, t);
    for (int i = 1, u, v, c; i <= m; ++i) {
        read(u, v, c);
        connect(u, v, c); connect(v, u, 0);
    }
    int ansf;
    dinic(s, t, ansf);
    printf("%lld\n", ansf);
    return 0;
}