网络流

• 这是初学网络流的时候从《算法竞赛进阶指南》抄下来的一份代码，自己理解的也不是很透彻。
• 注意，边要从 (1) 开始计，不然直接 (xor) 运算的话取反边会直接炸掉。

#include <bits/stdc++.h>
#define int long long

namespace Basic {
	template <typename Temp> inline void read(Temp & res) {
		Temp fh = 1; res = 0; char ch = getchar();
		for(; !isdigit(ch); ch = getchar()) if(ch == '-') fh = -1;
		for(; isdigit(ch); ch = getchar()) res = (res << 3) + (res << 1) + (ch ^ '0');
		res = res * fh; 
	}
	template <typename Temp> inline void Checkmax(Temp & res, Temp comp)  {if(comp > res) res = comp;} 
	template <typename Temp> inline void Checkmin(Temp & res, Temp comp)  {if(comp < res) res = comp;}
}

using namespace std;
using namespace Basic;

const int Maxn = 2e2 + 5;
const int Maxm = 5e3 + 5;
const int INF = 0x7fffffff >> 1;

struct e {
	int to, nxt, flow;
} b[Maxm << 1];
int head[Maxn], ecnt = 1;

inline void add(int u, int v, int f) {b[++ecnt] = (e){v, head[u], f}; head[u] = ecnt;}

int depth[Maxn], curr[Maxn];

int n, m, st, ed;

int Max_flow, last_flow;

queue<int> q;
inline bool bfs() {
	memset(depth, 0, sizeof(depth));
	while(!q.empty()) q.pop();
	depth[st] = 1; q.push(st); curr[st] = head[st];
	while(!q.empty()) {
		int tnow = q.front(); q.pop();
		for(register int i = head[tnow]; i; i = b[i].nxt) {
			int tto = b[i].to, tw = b[i].flow;
			if((!tw) || (depth[tto])) continue;
			q.push(tto);
			depth[tto] = depth[tnow] + 1; curr[tto] = head[tto];
			if(tto == ed) return true;
		}
	}
	return false;
}

int dfs(int t, int Flow) {
	if(t == ed) return Flow;
	int rest = Flow, k, i;
	for(i = curr[t]; i && rest; i = b[i].nxt) {
		int tto = b[i].to, tw = b[i].flow; curr[t] = i;
		if((tw == 0) || (depth[tto] != depth[t] + 1)) continue;
		k = dfs(tto, min(rest, tw));
		rest -= k;
		b[i].flow -= k;
		b[i ^ 1].flow += k;
		if(!k) depth[tto] == 0;
	}
	return Flow - rest;
}

signed main() {
	read(n); read(m); read(st); read(ed);
	int x, y, z;
	while(m--) {
		read(x); read(y); read(z);
		add(x, y, z);
		add(y, x, 0);
	}
	while(bfs()) {
		/*
		for(register int i = 1; i <= n; ++i) {
			printf("%d ", depth[i]); 
		}
		puts("");
		*/
		while(last_flow = dfs(st, INF)) Max_flow += last_flow;
	}
	printf("%d", Max_flow);
	return 0;
}