fix segmentation fault in trie c++ - c++

Question

I'm using a trie implementation to store and search words in c++ programming language. In using the search() function, I am getting a segmentation fault when searching for a specific word. It seems that the error occurred in checking if the struct is null.
here is the error message:
Program received signal SIGSEGV, Segmentation fault.
0x000055555555b2ff in search (this=0x55555577ee70,
wordlist=0x55555577ef00, word="a1g6os") at test.cc:30
if (!pCrawl->children[index])
here is the source code:
#include <bits/stdc++.h>
using namespace std;
const int ALPHABET_SIZE = 26;
struct TrieNode {
struct TrieNode *children[ALPHABET_SIZE];
bool isEndOfWord;
};
struct TrieNode *getNode(void) {
struct TrieNode *pNode = new TrieNode;
pNode->isEndOfWord = false;
for (int i = 0; i < ALPHABET_SIZE; i++)
pNode->children[i] = NULL;
return pNode;
}
void insert(struct TrieNode *root, string key) {
struct TrieNode *pCrawl = root;
for (int i = 0; i < key.length(); i++) {
int index = key[i] - 'a';
if (!pCrawl->children[index])
pCrawl->children[index] = getNode();
pCrawl = pCrawl->children[index];
}
// mark last node as leaf
pCrawl->isEndOfWord = true;
}
// Returns true if key presents in trie, else
// false
bool search(struct TrieNode *root, string key) {
struct TrieNode *pCrawl = root;
for (int i = 0; i < key.length(); i++) {
int index = key[i] - 'a';
if (!pCrawl->children[index])
return false;
pCrawl = pCrawl->children[index];
}
return (pCrawl != NULL && pCrawl->isEndOfWord);
}
int main() {
string keys[] = {"the", "a", "there",
"answer", "any", "by",
"bye", "their" };
int n = sizeof(keys)/sizeof(keys[0]);
struct TrieNode *root = getNode();
for (int i = 0; i < n; i++)
insert(root, keys[i]);
// Search for different keys
search(root, "a1g6os")? cout << "Yes\n" :
cout << "No\n";
return 0;
}

Answer 1

Both #Some programmer dude and #JohnnyJohansson have pointed out the root cause. The live test showed where the code read the array out-of-bounds. Actually the fix is easy once you understand what happens. The following is the fixed code if you cannot figure it out by yourself. A live test of it is here cee.studio
#include<iostream>
using namespace std;
const int ALPHABET_SIZE = 75; // increase the range
struct TrieNode {
struct TrieNode *children[ALPHABET_SIZE];
bool isEndOfWord;
};
struct TrieNode *getNode(void) {
struct TrieNode *pNode = new TrieNode;
pNode->isEndOfWord = false;
for (int i = 0; i < ALPHABET_SIZE; i++)
pNode->children[i] = NULL;
return pNode;
}
void insert(struct TrieNode *root, string key) {
struct TrieNode *pCrawl = root;
for (int i = 0; i < key.length(); i++) {
int index = key[i] - '0'; // lower the low bound
if (!pCrawl->children[index])
pCrawl->children[index] = getNode();
pCrawl = pCrawl->children[index];
}
// mark last node as leaf
pCrawl->isEndOfWord = true;
}
// Returns true if key presents in trie, else
// false
bool search(struct TrieNode *root, string key) {
struct TrieNode *pCrawl = root;
for (int i = 0; i < key.length(); i++) {
int index = key[i] - '0'; // lower the low bound
if (!pCrawl->children[index])
return false;
pCrawl = pCrawl->children[index];
}
return (pCrawl != NULL && pCrawl->isEndOfWord);
}
int main() {
string keys[] = {"the", "a", "there",
"answer", "any", "by",
"bye", "their" };
int n = sizeof(keys)/sizeof(keys[0]);
struct TrieNode *root = getNode();
for (int i = 0; i < n; i++)
insert(root, keys[i]);
// Search for different keys
search(root, "a1g6os")? cout << "Yes\n" :
cout << "No\n";
return 0;
}