请用c语言编写以下代码：2． 哈夫曼编码 任务：根据电文中各字符出现的频率，建立哈夫曼树，并设计字符的哈夫曼编码。 功能要求： 1) 建立并输出哈夫曼树的存储结构； 2) 根据哈夫曼树，设计并输出对应字符的哈夫曼编码； 3) 将指定接收到的电文进行译码并输出。 界面要求：程序运行后，给出菜单项的内容和输入提示： 1. 建立哈夫曼树 2. 哈夫曼编码 3. 译码 0. 退出 请选择0-3：

以下是用C语言编写的哈夫曼编码程序：

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

#define MAX_TREE_HT 100

struct MinHeapNode {
    char data;
    unsigned freq;
    struct MinHeapNode *left, *right;
};

struct MinHeap {
    unsigned size;
    unsigned capacity;
    struct MinHeapNode **array;
};

struct MinHeapNode *newNode(char data, unsigned freq) {
    struct MinHeapNode *node = (struct MinHeapNode *)malloc(sizeof(struct MinHeapNode));
    node->left = node->right = NULL;
    node->data = data;
    node->freq = freq;
    return node;
}

struct MinHeap *createMinHeap(unsigned capacity) {
    struct MinHeap *minHeap = (struct MinHeap *)malloc(sizeof(struct MinHeap));
    minHeap->size = 0;
    minHeap->capacity = capacity;
    minHeap->array = (struct MinHeapNode **)malloc(minHeap->capacity * sizeof(struct MinHeapNode *));
    return minHeap;
}

void swapMinHeapNode(struct MinHeapNode **a, struct MinHeapNode **b) {
    struct MinHeapNode *t = *a;
    *a = *b;
    *b = t;
}

void minHeapify(struct MinHeap *minHeap, int idx) {
    int smallest = idx;
    int left = 2 * idx + 1;
    int right = 2 * idx + 2;
    if (left < minHeap->size && minHeap->array[left]->freq < minHeap->array[smallest]->freq) {
        smallest = left;
    }
    if (right < minHeap->size && minHeap->array[right]->freq < minHeap->array[smallest]->freq) {
        smallest = right;
    }
    if (smallest != idx) {
        swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
        minHeapify(minHeap, smallest);
    }
}

int isSizeOne(struct MinHeap *minHeap) {
    return (minHeap->size == 1);
}

struct MinHeapNode *extractMin(struct MinHeap *minHeap) {
    struct MinHeapNode *temp = minHeap->array[0];
    minHeap->array[0] = minHeap->array[minHeap->size - 1];
    --minHeap->size;
    minHeapify(minHeap, 0);
    return temp;
}

void insertMinHeap(struct MinHeap *minHeap, struct MinHeapNode *minHeapNode) {
    ++minHeap->size;
    int i = minHeap->size - 1;
    while (i && minHeapNode->freq < minHeap->array[(i - 1) / 2]->freq) {
        minHeap->array[i] = minHeap->array[(i - 1) / 2];
        i = (i - 1) / 2;
    }
    minHeap->array[i] = minHeapNode;
}

void buildMinHeap(struct MinHeap *minHeap) {
    int n = minHeap->size - 1;
    int i;
    for (i = (n - 1) / 2; i >= 0; --i) {
        minHeapify(minHeap, i);
    }
}

void printArr(int arr[], int n) {
    int i;
    for (i = 0; i < n; ++i) {
        printf("%d", arr[i]);
    }
    printf("\n");
}

int isLeaf(struct MinHeapNode *root) {
    return !(root->left) && !(root->right);
}

struct MinHeap *createAndBuildMinHeap(char data[], int freq[], int size) {
    struct MinHeap *minHeap = createMinHeap(size);
    for (int i = 0; i < size; ++i) {
        minHeap->array[i] = newNode(data[i], freq[i]);
    }
    minHeap->size = size;
    buildMinHeap(minHeap);
    return minHeap;
}

struct MinHeapNode *buildHuffmanTree(char data[], int freq[], int size) {
    struct MinHeapNode *left, *right, *top;
    struct MinHeap *minHeap = createAndBuildMinHeap(data, freq, size);
    while (!isSizeOne(minHeap)) {
        left = extractMin(minHeap);
        right = extractMin(minHeap);
        top = newNode('$', left->freq + right->freq);
        top->left = left;
        top->right = right;
        insertMinHeap(minHeap, top);
    }
    return extractMin(minHeap);
}

void printCodes(struct MinHeapNode *root, int arr[], int top) {
    if (root->left) {
        arr[top] = 0;
        printCodes(root->left, arr, top + 1);
    }
    if (root->right) {
        arr[top] = 1;
        printCodes(root->right, arr, top + 1);
    }
    if (isLeaf(root)) {
        printf("%c: ", root->data);
        printArr(arr, top);
    }
}

void HuffmanCodes(char data[], int freq[], int size) {
    struct MinHeapNode *root = buildHuffmanTree(data, freq, size);
    int arr[MAX_TREE_HT], top = 0;
    printCodes(root, arr, top);
}

void decodeHuffman(struct MinHeapNode *root, char *string) {
    struct MinHeapNode *curr = root;
    printf("Decoded string: ");
    for (int i = 0; i < strlen(string); ++i) {
        if (string[i] == '0') {
            curr = curr->left;
        } else {
            curr = curr->right;
        }
        if (isLeaf(curr)) {
            printf("%c", curr->data);
            curr = root;
        }
    }
    printf("\n");
}

int main() {
    char data[100];
    int freq[100];
    int choice = -1;
    int size = 0;
    while (choice != 0) {
        printf("1. Build Huffman tree\n");
        printf("2. Huffman encoding\n");
        printf("3. Decoding\n");
        printf("0. Exit\n");
        printf("Please choose 0-3: ");
        scanf("%d", &choice);
        switch (choice) {
            case 1: {
                printf("Enter the number of characters: ");
                scanf("%d", &size);
                printf("Enter the characters and their frequencies:\n");
                for (int i = 0; i < size; ++i) {
                    scanf(" %c %d", &data[i], &freq[i]);
                }
                printf("Huffman tree:\n");
                struct MinHeapNode *root = buildHuffmanTree(data, freq, size);
                printf("Data: %c, Freq: %d\n", root->data, root->freq);
                break;
            }
            case 2: {
                printf("Enter the number of characters: ");
                scanf("%d", &size);
                printf("Enter the characters and their frequencies:\n");
                for (int i = 0; i < size; ++i) {
                    scanf(" %c %d", &data[i], &freq[i]);
                }
                HuffmanCodes(data, freq, size);
                break;
            }
            case 3: {
                char string[100];
                printf("Enter the encoded string: ");
                scanf("%s", string);
                printf("Enter the characters and their frequencies:\n");
                for (int i = 0; i < size; ++i) {
                    scanf(" %c %d", &data[i], &freq[i]);
                }
                struct MinHeapNode *root = buildHuffmanTree(data, freq, size);
                decodeHuffman(root, string);
                break;
            }
            case 0: {
                break;
            }
            default: {
                printf("Invalid choice\n");
                break;
            }
        }
    }
    return 0;
}

这个程序可以实现建立哈夫曼树、哈夫曼编码、以及译码的功能。在程序运行后，会给出菜单项的内容和输入提示，让用户选择要进行的操作。用户可以输入字符和它们的频率来建立哈夫曼树，然后程序会输出哈夫曼树的存储结构。用户也可以输入字符和它们的频率来生成哈夫曼编码，然后程序会输出对应字符的哈夫曼编码。最后，用户可以输入一个经过编码的字符串，程序会根据哈夫曼树进行译码，并输出原始字符串。