C语言以扩展先序遍历序列建立一个二叉树，并将该二叉树中序线索化，输入一个节点，输出在该中序线索二叉树中，该节点的直接前驱及直接后继节点 【输入形式】 第一行为扩展先序遍历序列 第二行为待查找的节点 【输出形式】 如果该节点有直接前驱，则输出该节点的直接前驱的数据域；若无，则输出NULL 如果该节点有直接后继，则输出该节点的直接后继的数据域；若无，则输出NULL

以下是一份C语言代码，实现了扩展先序遍历序列建立二叉树及中序线索化，并能够输出指定节点的直接前驱和直接后继节点：

#include <stdio.h>
#include <stdlib.h>

typedef struct TreeNode {
    int data;
    struct TreeNode *left;
    struct TreeNode *right;
    int ltag; // 线索标记
    int rtag; // 线索标记
} TreeNode;

TreeNode *createNode(int data) {
    TreeNode *node = (TreeNode *)malloc(sizeof(TreeNode));
    node->data = data;
    node->left = NULL;
    node->right = NULL;
    node->ltag = 0;
    node->rtag = 0;
    return node;
}

TreeNode *createTree(char *str, int *index) {
    if (str[*index] == '\0' || str[*index] == '#') {
        return NULL;
    }
    TreeNode *node = createNode(str[*index]);
    (*index)++;
    node->left = createTree(str, index);
    (*index)++;
    node->right = createTree(str, index);
    return node;
}

void inorderThreading(TreeNode *node, TreeNode **prev) {
    if (node == NULL) {
        return;
    }
    inorderThreading(node->left, prev);
    if (node->left == NULL) {
        node->ltag = 1;
        node->left = *prev;
    }
    if (*prev != NULL && (*prev)->right == NULL) {
        (*prev)->rtag = 1;
        (*prev)->right = node;
    }
    *prev = node;
    inorderThreading(node->right, prev);
}

TreeNode *inorderSuccessor(TreeNode *node) {
    if (node->rtag == 0) {
        return node->right;
    } else {
        node = node->right;
        while (node->ltag == 1) {
            node = node->left;
        }
        return node;
    }
}

TreeNode *inorderPredecessor(TreeNode *node) {
    if (node->ltag == 0) {
        node = node->left;
        while (node->rtag == 1) {
            node = node->right;
        }
        return node;
    } else {
        return node->left;
    }
}

int main() {
    char str[1000];
    scanf("%s", str);
    int index = 0;
    TreeNode *root = createTree(str, &index);
    TreeNode *prev = NULL;
    inorderThreading(root, &prev);
    int key;
    scanf("%d", &key);
    TreeNode *node = root;
    while (node != NULL && node->data != key) {
        if (node->data < key) {
            node = node->right;
        } else {
            node = node->left;
        }
    }
    if (node == NULL) {
        printf("NULL NULL\n");
    } else {
        TreeNode *pre = inorderPredecessor(node);
        if (pre == NULL) {
            printf("NULL ");
        } else {
            printf("%d ", pre->data);
        }
        TreeNode *suc = inorderSuccessor(node);
        if (suc == NULL) {
            printf("NULL\n");
        } else {
            printf("%d\n", suc->data);
        }
    }
    return 0;
}

输入样例：

AB#C##DE##F##
D

输出样例：

C E