Problem p02561 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;

const int inf = 0x3f3f3f3f;

struct edge {
  int to, cap;
  char flag; // 0: r -> l   1: u -> d
  edge() : flag(-1) {}
  edge(const int &t, const int &c, const char &f) : to(t), cap(c), flag(f) {}
} ge[60000];
int n, m;
vector<int> gl[10010];
int depth[10010], q[10010];
int iter[10010];

int N, M;
char a[110][110];
char dir[4][2] = {1, 0, 0, 1, -1, 0, 0, -1};
char chr[4][2] = {'v', '^', '>', '<', '^', 'v', '<', '>'};
void AddEdge(const int &from, const int &to, const int &cap, const char &flag);
int MaxFlow(const int &from, const int &to);
int dfs(const int &from, const int &to, const int &flow);
void GetAns(int from, int to, char flag);

int main() {
  scanf("%d%d", &N, &M);
  const int s = N * M, t = N * M + 1;
  for (int i = 0; i < N; ++i)
    scanf("%s", a[i]);

  for (int i = 0; i < N; ++i)
    for (int j = i & 1; j < M; j += 2)
      if (a[i][j] == '.') {
        if (i + 1 < N && a[i + 1][j] == '.')
          AddEdge(i * M + j, (i + 1) * M + j, 1, 0);
        if (j + 1 < M && a[i][j + 1] == '.')
          AddEdge(i * M + j, i * M + j + 1, 1, 1);
        if (i > 0 && a[i - 1][j] == '.')
          AddEdge(i * M + j, (i - 1) * M + j, 1, 2);
        if (j > 0 && a[i][j - 1] == '.')
          AddEdge(i * M + j, i * M + j - 1, 1, 3);
      }
  for (int i = 0; i < N; ++i)
    for (int j = 0; j < M; ++j)
      if (a[i][j] == '.')
        if (i + j & 1)
          AddEdge(i * M + j, t, a[i][j] == '.', -1);
        else
          AddEdge(s, i * M + j, a[i][j] == '.', -1);
  printf("%d\\n", MaxFlow(s, t));

  for (int i = 0; ge[i].flag != -1; i += 2)
    if (!ge[i].cap)
      GetAns(ge[i ^ 1].to, ge[i].to, ge[i].flag);
  for (int i = 0; i < N; ++i)
    printf("%s\\n", a[i]);
  return 0;
}

void GetAns(int from, int to, char flag) {
  a[from / M][from % M] = chr[flag][0], a[to / M][to % M] = chr[flag][1];
}

void AddEdge(const int &from, const int &to, const int &cap, const char &flag) {
  ge[m] = edge(to, cap, flag);
  gl[from].push_back(m++);
  ge[m] = edge(from, 0, -1);
  gl[to].push_back(m++);
}

int MaxFlow(const int &from, const int &to) {
  int flow = 0;
  while (true) {
    memset(depth, -1, sizeof(depth));
    depth[from] = 0;
    int h = 0, t = 1;
    q[h] = from;
    while (h < t) {
      int u = q[h++];
      for (int i = 0; i < gl[u].size(); ++i) {
        edge &e = ge[gl[u][i]];
        if (e.cap > 0 && depth[e.to] == -1)
          q[t++] = e.to, depth[e.to] = depth[u] + 1;
      }
    }
    if (depth[to] == -1)
      return flow;

    memset(iter, 0, sizeof(iter));
    int f;
    while (f = dfs(from, to, inf))
      flow += f;
  }
}

int dfs(const int &from, const int &to, const int &flow) {
  if (from == to)
    return flow;
  for (int &i = iter[from]; i < gl[from].size(); ++i) {
    edge &e = ge[gl[from][i]], &e1 = ge[gl[from][i] ^ 1];
    if (e.cap > 0 && depth[e.to] == depth[from] + 1) {
      int f = dfs(e.to, to, min(flow, e.cap));
      if (f) {
        e.cap -= f;
        e1.cap += f;
        return f;
      }
    }
  }
  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

const int inf = 0x3f3f3f3f;

struct edge {
  int to, cap;
  char flag; // 0: r -> l   1: u -> d
  edge() {}
  edge(const int &t, const int &c, const char &f) : to(t), cap(c), flag(f) {}
} ge[60000];
int n, m;
vector<int> gl[10010];
int depth[10010], q[10010];
int iter[10010];

int N, M;
char a[110][110];
char dir[4][2] = {1, 0, 0, 1, -1, 0, 0, -1};
char chr[4][2] = {'v', '^', '>', '<', '^', 'v', '<', '>'};
void AddEdge(const int &from, const int &to, const int &cap, const char &flag);
int MaxFlow(const int &from, const int &to);
int dfs(const int &from, const int &to, const int &flow);
inline void GetAns(const int &from, const int &to, const char &flag);

int main() {
  scanf("%d%d", &N, &M);
  const int s = N * M, t = N * M + 1;
  for (int i = 0; i < N; ++i)
    scanf("%s", a[i]);

  for (int i = 0; i < N; ++i)
    for (int j = i & 1; j < M; j += 2)
      if (a[i][j] == '.') {
        if (i + 1 < N && a[i + 1][j] == '.')
          AddEdge(i * M + j, (i + 1) * M + j, 1, 0);
        if (j + 1 < M && a[i][j + 1] == '.')
          AddEdge(i * M + j, i * M + j + 1, 1, 1);
        if (i > 0 && a[i - 1][j] == '.')
          AddEdge(i * M + j, (i - 1) * M + j, 1, 2);
        if (j > 0 && a[i][j - 1] == '.')
          AddEdge(i * M + j, i * M + j - 1, 1, 3);
      }
  for (int i = 0; i < N; ++i)
    for (int j = 0; j < M; ++j)
      if (a[i][j] == '.')
        if (i + j & 1)
          AddEdge(i * M + j, t, a[i][j] == '.', -1);
        else
          AddEdge(s, i * M + j, a[i][j] == '.', -1);
  printf("%d\\n", MaxFlow(s, t));

  for (int i = 0; ge[i].flag != -1; i += 2)
    if (!ge[i].cap)
      GetAns(ge[i ^ 1].to, ge[i].to, ge[i].flag);
  for (int i = 0; i < N; ++i)
    printf("%s\\n", a[i]);
  return 0;
}

inline void GetAns(const int &from, const int &to, const char &flag) {
  a[from / M][from % M] = chr[flag][0], a[to / M][to % M] = chr[flag][1];
}

void AddEdge(const int &from, const int &to, const int &cap, const char &flag) {
  ge[m] = edge(to, cap, flag);
  gl[from].push_back(m++);
  ge[m] = edge(from, 0, -1);
  gl[to].push_back(m++);
}

int MaxFlow(const int &from, const int &to) {
  int flow = 0;
  while (true) {
    memset(depth, -1, sizeof(depth));
    depth[from] = 0;
    int h = 0, t = 1;
    q[h] = from;
    while (h < t) {
      int u = q[h++];
      for (int i = 0; i < gl[u].size(); ++i) {
        edge &e = ge[gl[u][i]];
        if (e.cap > 0 && depth[e.to] == -1)
          q[t++] = e.to, depth[e.to] = depth[u] + 1;
      }
    }
    if (depth[to] == -1)
      return flow;

    memset(iter, 0, sizeof(iter));
    int f;
    while (f = dfs(from, to, inf))
      flow += f;
  }
}

int dfs(const int &from, const int &to, const int &flow) {
  if (from == to)
    return flow;
  for (int &i = iter[from]; i < gl[from].size(); ++i) {
    edge &e = ge[gl[from][i]], &e1 = ge[gl[from][i] ^ 1];
    if (e.cap > 0 && depth[e.to] == depth[from] + 1) {
      int f = dfs(e.to, to, min(flow, e.cap));
      if (f) {
        e.cap -= f;
        e1.cap += f;
        return f;
      }
    }
  }
  return 0;
}