在K&R中找到的C问题中的二叉树实现

struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
};

struct TreeNode* insert(struct TreeNode* root, int val) {
    if (root == NULL) {
        root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
        root->val = val;
        root->left = NULL;
        root->right = NULL;
    } else if (val< root->val) {
        root->left = insert(root->left, val);
    } else {
        root->right = insert(root->right, val);
    }
    return root;
}

struct TreeNode* delete(struct TreeNode* root, int val) {
    if (root == NULL) {
        return NULL;
    }
    if (val< root->val) {
        root->left = delete(root->left, val);
    } else if (val > root->val) {
        root->right = delete(root->right, val);
    } else {
        if (root->left == NULL) {
            struct TreeNode* temp = root->right;
            free(root);
            return temp;
        } else if (root->right == NULL) {
            struct TreeNode* temp = root->left;
            free(root);
            return temp;
        }
        struct TreeNode* temp = minValueNode(root->right);
        root->val = temp->val;
        root->right = delete(root->right, temp->val);
    }
    return root;
}

struct TreeNode* search(struct TreeNode* root, int val) {
    if (root == NULL || root->val == val) {
        return root;
    }
    if (val< root->val) {
        return search(root->left, val);
    }
    return search(root->right, val);
}

void preOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    printf("%d ", root->val);
    preOrder(root->left);
    preOrder(root->right);
}

void inOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    inOrder(root->left);
    printf("%d ", root->val);
    inOrder(root->right);
}

void postOrder(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    postOrder(root->left);
    postOrder(root->right);
    printf("%d ", root->val);
}