Problem p02285 (C++)

Buggy submission (status: Runtime Error):
#include <cstdio>
#include <iostream>

using namespace std;

struct Node {
  int key;
  Node *parent, *left, *right;
};

Node *root, *NIL;

void insert(int k) {
  Node *y = NIL;
  Node *x = root;
  Node *z;

  z = new Node;
  z->key = k;
  z->left = NIL;
  z->right = NIL;
  while (x != NIL) {
    y = x;
    if (z->key < x->key) {
      x = x->left;
    } else {
      x = x->right;
    }
  }

  z->parent = y;
  if (y == NIL) {
    root = z;
  } else {
    if (z->key < y->key)
      y->left = z;
    else
      y->right = z;
  }
}

Node *find(Node *u, int k) {
  while (u != NIL && k != u->key) {
    if (k < u->key)
      u = u->left;
    else
      u = u->right;
  }
  return u;
}

Node *treeMinimum(Node *x) {
  while (x->left != NIL)
    x = x->left;
  return x;
}

Node *treeSuccessor(Node *x) {
  if (x->right != NIL)
    return treeMinimum(x->right);
  Node *y = x->parent;
  while (y != NIL && x == y->right) {
    x = y;
    y = y->parent;
  }
  return y;
}

void treeDelete(Node *z) {
  Node *y;
  Node *x;

  if (z->left == NIL || z->right)
    y = z;
  else
    y = treeSuccessor(z);

  if (y->left != NIL) {
    x = y->left;
  } else {
    x = y->right;
  }
  if (x != NIL)
    x->parent = y->parent;

  if (y->parent == NIL) {
    root = x;
  } else {
    if (y == y->parent->left)
      y->parent->left = x;
    else
      y->parent->right = x;
  }

  if (y != z)
    z->key = y->key;

  delete y;
}

void inorder(Node *u) {
  if (u == NIL)
    return;
  inorder(u->left);
  printf(" %d", u->key);
  inorder(u->right);
}
void preorder(Node *u) {
  if (u == NIL)
    return;
  printf(" %d", u->key);
  preorder(u->left);
  preorder(u->right);
}

int main() {
  int n, x;
  string com;

  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> com;
    if (com == "insert") {
      cin >> x;
      insert(x);
    } else if (com == "print") {
      inorder(root);
      cout << endl;
      preorder(root);
      cout << endl;
    } else if (com == "find") {
      cin >> x;
      if (find(root, x) != NIL)
        cout << "yes" << endl;
      else
        cout << "no" << endl;

    } else if (com == "delete") {
      cin >> x;
      treeDelete(find(root, x));
    }
  }

  return 0;
}

Fixed submission:
#include <cstdio>
#include <iostream>

using namespace std;

struct Node {
  int key;
  Node *parent, *left, *right;
};

Node *root, *NIL;

void insert(int k) {
  Node *y = NIL;
  Node *x = root;
  Node *z;

  z = new Node;
  z->key = k;
  z->left = NIL;
  z->right = NIL;
  while (x != NIL) {
    y = x;
    if (z->key < x->key) {
      x = x->left;
    } else {
      x = x->right;
    }
  }

  z->parent = y;
  if (y == NIL) {
    root = z;
  } else {
    if (z->key < y->key)
      y->left = z;
    else
      y->right = z;
  }
}

Node *find(Node *u, int k) {
  while (u != NIL && k != u->key) {
    if (k < u->key)
      u = u->left;
    else
      u = u->right;
  }
  return u;
}

Node *treeMinimum(Node *x) {
  while (x->left != NIL)
    x = x->left;
  return x;
}

Node *treeSuccessor(Node *x) {
  if (x->right != NIL)
    return treeMinimum(x->right);
  Node *y = x->parent;
  while (y != NIL && x == y->right) {
    x = y;
    y = y->parent;
  }
  return y;
}

void treeDelete(Node *z) {
  Node *y;
  Node *x;

  if (z->left == NIL || z->right == NIL)
    y = z;
  else
    y = treeSuccessor(z);

  if (y->left != NIL) {
    x = y->left;
  } else {
    x = y->right;
  }
  if (x != NIL)
    x->parent = y->parent;

  if (y->parent == NIL) {
    root = x;
  } else {
    if (y == y->parent->left)
      y->parent->left = x;
    else
      y->parent->right = x;
  }

  if (y != z)
    z->key = y->key;

  delete y;
}

void inorder(Node *u) {
  if (u == NIL)
    return;
  inorder(u->left);
  printf(" %d", u->key);
  inorder(u->right);
}
void preorder(Node *u) {
  if (u == NIL)
    return;
  printf(" %d", u->key);
  preorder(u->left);
  preorder(u->right);
}

int main() {
  int n, x;
  string com;

  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> com;
    if (com == "insert") {
      cin >> x;
      insert(x);
    } else if (com == "print") {
      inorder(root);
      cout << endl;
      preorder(root);
      cout << endl;
    } else if (com == "find") {
      cin >> x;
      if (find(root, x) != NIL)
        cout << "yes" << endl;
      else
        cout << "no" << endl;

    } else if (com == "delete") {
      cin >> x;
      treeDelete(find(root, x));
    }
  }

  return 0;
}