Problem p02285 (C++) — buggy submission, verdict: Runtime Error

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
using namespace std;

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

Node *root, *NIL; // 宣言時ポインタにはNULL(0x0)が入っているため,
                  // 木の根が設定されていなければroot == NILとなる.

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

  z = (Node *)malloc(sizeof(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;
}

void deleteNode(int k) {
  Node *y;
  Node *z = find(root, k);
  if (z->left == NIL && z->right == NIL) {
    if (z->parent->left == z)
      z->parent->left = NIL;
    else
      z->parent->right = NIL;
    free(z);
  } else if (z->left != NIL && z->right == NIL) {
    if (z->parent->left == z)
      z->parent->left = z->left;
    else
      z->parent->right = z->left;
    z->left->parent = z->parent;
    free(z);
  } else if (z->left == NIL && z->right != NIL) {
    if (z->parent->left == z)
      z->parent->left = z->right;
    else
      z->parent->right = z->right;
    z->right->parent = z->parent;
    free(z);
  } else {
    y = z->right;
    while (y->left != NIL)
      y = y->left;
    z->key = y->key;
    if (y->parent->left == y)
      y->parent->left = NIL;
    else
      y->parent->right = NIL;
    free(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, i, x;
  string com;

  scanf("%d", &n);

  for (i = 0; i < n; i++) {
    cin >> com;
    if (com == "insert") {
      scanf("%d", &x);
      insert(x);
    } else if (com == "print") {
      inorder(root);
      printf("\\n");
      preorder(root);
      printf("\\n");
    } else if (com == "find") {
      scanf("%d", &x);
      Node *t = find(root, x);
      if (t != NIL)
        printf("yes\\n");
      else
        printf("no\\n");
    } else {
      scanf("%d", &x);
      deleteNode(x);
    }
  }

  return 0;
}


Error: 0

Fixed submission:

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
using namespace std;

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

Node *root, *NIL; // 宣言時ポインタにはNULL(0x0)が入っているため,
                  // 木の根が設定されていなければroot == NILとなる.

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

  z = (Node *)malloc(sizeof(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;
}

void deleteNode(int k) {
  Node *y;
  Node *z = find(root, k);
  if (z->left == NIL && z->right == NIL) {
    if (z->parent->left == z)
      z->parent->left = NIL;
    else
      z->parent->right = NIL;
    free(z);
  } else if (z->left != NIL && z->right == NIL) {
    if (z->parent->left == z)
      z->parent->left = z->left;
    else
      z->parent->right = z->left;
    z->left->parent = z->parent;
    free(z);
  } else if (z->left == NIL && z->right != NIL) {
    if (z->parent->left == z)
      z->parent->left = z->right;
    else
      z->parent->right = z->right;
    z->right->parent = z->parent;
    free(z);
  } else {
    y = z->right;
    while (y->left != NIL)
      y = y->left;
    z->key = y->key;
    if (y->parent->left == y) {
      y->parent->left = y->right;
    } else {
      y->parent->right = y->right;
    }
    y->right->parent = y->parent;
    free(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, i, x;
  string com;

  scanf("%d", &n);

  for (i = 0; i < n; i++) {
    cin >> com;
    if (com == "insert") {
      scanf("%d", &x);
      insert(x);
    } else if (com == "print") {
      inorder(root);
      printf("\\n");
      preorder(root);
      printf("\\n");
    } else if (com == "find") {
      scanf("%d", &x);
      Node *t = find(root, x);
      if (t != NIL)
        printf("yes\\n");
      else
        printf("no\\n");
    } else {
      scanf("%d", &x);
      deleteNode(x);
    }
  }

  return 0;
}