在C中的二叉树中递归搜索

Question

The function receives parameter node (has member name) and str (the name to search)

{
    if (node == NULL) return NULL;
    if (strcmp(node->name, str) == 0) return node;
    node = search_RtLR(node->left, str);
    if (node != NULL) return node;
    node = search_RtLR(node->right, str);
    if (node != NULL) return node;

    return NULL;
}

When I search a name that is in left subtree, it works, but when I search in right subtree the program terminates(also when there is no such name in the tree), and I can't find what's wrong. The tree is not sorted in alphabetical order.

Answer 1

You're overwriting your node parameter variable:

node = search_RtLR(node->left, str); // overwriting node here at assignment
if (node != NULL) return node;
node = search_RtLR(node->right, str); // node is NULL here, look at line above!

You shouldn't!

Defining your parameters as const (since this is a function that does not change any data) also helps (as the compiler will warn you if you try to overwrite the const variables):

Node* search_RtLR(const Node* node, const char* str) {
    if (node == NULL) return NULL;
    if (strcmp(node->name, str) == 0) return node; 
    const Node* newNode = search_RtLR(node->left, str);
    if (newNode != NULL) return newNode;
    return search_RtLR(node->right, str);
}

Answer 2

When the string is not in the leftsubtree the recursive search returns NULL, which you assign to node . Then search_RtLR(node->right, str) searches 'nowhere'. You should not overwrite your node :

if (node == NULL) return NULL;
if (strcmp(node->name, str) == 0) return node;
node1 = search_RtLR(node->left, str);
if (node1 != NULL) return node1;
node1 = search_RtLR(node->right, str);
if (node1 != NULL) return node1;

return NULL;