逃脫問題，在算法導論網絡流后面的習題里面有提到，就是一個點不相交的路徑，要拆點，導致圖很大，我的鄰接矩陣isap超時，換成鄰接表還是超時。。
就拿人家的代碼交了，思想還是簡單的。我的網路流模板當圖一大就超時。。真杯具。
#include <iostream>
#include <vector>
#include <queue>
// 50 * 50 * 2 + 2 = 5002
#define MAX 5002
#define RESIDUE(s,t) (capacity[s][t]-flow[s][t])
using namespace std;
int dr[4] = {0, 1, 0, -1};
int dc[4] = {1, 0, -1, 0};
int bank[MAX][2];
int flow[MAX][MAX];
int capacity[MAX][MAX];
int s, a, b;
int prev[MAX];
inline int getSourceID() { return s * a * 2; }
inline int getDestinationID() { return s * a * 2 + 1; }
inline int getIDByRowCol(int r, int c, int rank) { return r * a + c + rank * s * a; }
inline void getRowColByID(int id, int &row, int &col, int &rank) {
rank = id >= s * a; id %= s * a;
row = id / a;
col = id % a;
}
inline bool isValid(int r, int c) { return 0 <= r && r < s && 0 <= c && c < a; }
inline bool isOnEdge(int r, int c) { return r == 0 || r == s - 1 || c == 0 || c == a - 1; }
void display(int id) {
if (prev[id] != id) {
display(prev[id]);
}
if (id == getSourceID()) {
cout << "source" << endl;
} else if (id == getDestinationID()) {
cout << "destination" << endl;
} else {
int row, col, rank;
getRowColByID(id, row, col, rank);
cout << row + 1 << " " << col + 1;
if (rank) cout << " shadow";
cout << endl;
}
}
main () {
int p;
cin >> p;
for (int P = 0; P < p; ++P) {
// Input
cin >> s >> a >> b;
for (int i = 0; i < b; ++i) {
cin >> bank[i][0] >> bank[i][1];
bank[i][0]--, bank[i][1]--;
}
// Initialize
memset(capacity, 0, sizeof(capacity));
memset(flow, 0, sizeof(flow));
// Source to banks
for (int i = 0; i < b; ++i) {
capacity[getSourceID()][getIDByRowCol(bank[i][0], bank[i][1], 0)] = 1;
}
// Adjacent grid
for (int i = 0; i < s; ++i) {
for (int j = 0; j < a; ++j) {
int fromID = getIDByRowCol(i, j, 0);
int _fromID = getIDByRowCol(i, j, 1);
capacity[_fromID][fromID] = 1;
for (int k = 0; k < 4; ++k) {
int ni = i + dr[k];
int nj = j + dc[k];
if (!isValid(ni, nj)) continue;
int toID = getIDByRowCol(ni, nj, 1);
capacity[fromID][toID] = 1;
}
}
}
// Grids on edge to destination
for (int i = 0; i < s; ++i) {
capacity[getIDByRowCol(i, 0, 0)][getDestinationID()] = 1;
capacity[getIDByRowCol(i, a - 1, 0)][getDestinationID()] = 1;
}
for (int j = 0; j < a; ++j) {
capacity[getIDByRowCol(0, j, 0)][getDestinationID()] = 1;
capacity[getIDByRowCol(s - 1, j, 0)][getDestinationID()] = 1;
}
// If a bank exists on a corner, transition between
// surface and shadow is forbidden.
for (int i = 0; i < b; ++i) {
capacity[getIDByRowCol(bank[i][0], bank[i][1], 1)][getIDByRowCol(bank[i][0], bank[i][1], 0)] = 0;
}
// Maxflow
int total = 0;
while (1) {
queue<int> que; que.push(getSourceID());
memset(prev, -1, sizeof(prev)); prev[getSourceID()] = getSourceID();
while (!que.empty() && prev[getDestinationID()] == -1) {
int u = que.front(); que.pop();
vector<int> nextID;
if (u == getSourceID()) {
for (int i = 0; i < b; ++i) {
//cout << bank[i][0] << " " << bank[i][1] << ", id = " << getIDByRowCol(bank[i][0], bank[i][1], 0) << endl;
nextID.push_back(getIDByRowCol(bank[i][0], bank[i][1], 0));
}
} else {
int row, col, rank;
getRowColByID(u, row, col, rank);
//cout << u << " " << row << " " << col << " " << rank << endl;
if (rank == 0 && isOnEdge(row, col)) {
nextID.push_back(getDestinationID());
} else if (rank == 1) {
nextID.push_back(getIDByRowCol(row, col, 0));
} else {
for (int i = 0; i < 4; ++i) {
int nrow = row + dr[i];
int ncol = col + dc[i];
if (!isValid(nrow, ncol)) continue;
nextID.push_back(getIDByRowCol(nrow, ncol, 1));
}
}
}
for (int i = 0; i < nextID.size(); ++i) {
int v = nextID[i];
if (prev[v] == -1 && RESIDUE(u, v) > 0) {
prev[v] = u;
que.push(v);
}
}
}
if (prev[getDestinationID()] == -1) break;
int inc = INT_MAX;
for (int j = getDestinationID(); prev[j] != j; j = prev[j]) {
inc = min(inc, RESIDUE(prev[j], j));
}
for (int j = getDestinationID(); prev[j] != j; j = prev[j]) {
flow[prev[j]][j] += inc, flow[j][prev[j]] -= inc;
}
//display(getDestinationID()); cout << endl;
total += inc;
}
//cout << total << endl;
if (total == b) {
cout << "possible" << endl;
} else {
cout << "not possible" << endl;
}
}
}