输入n个结点的权值，用c语言构建哈夫曼树，请给出代码及样例输入及输出。

在C语言中，构建哈夫曼树并输出编码，可以采用递归的方式来实现。由于哈夫曼树是一种特殊的二叉树，我们首先需要定义一个`HuffmanNode`结构来存储节点信息，并实现比较和合并的功能。以下是简化版的代码示例：

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

typedef struct {
    char data;
    int freq;
    HuffmanNode *left, *right;
} HuffmanNode;

// 合并两个频率最小的节点
HuffmanNode* merge(HuffmanNode* node1, HuffmanNode* node2) {
    if (node1 == NULL)
        return node2;
    if (node2 == NULL)
        return node1;

    if (node1->freq < node2->freq) {
        node1->right = merge(node1->right, node2);
        return node1;
    } else {
        node2->left = merge(node1, node2->left);
        return node2;
    }
}

// 插入节点到堆
void insert(HuffmanNode** heap, HuffmanNode* newNode) {
    if (*heap == NULL) {
        *heap = newNode;
        return;
    }

    HuffmanNode* last = *heap;
    while (last->freq < newNode->freq || (last->freq == newNode->freq && last->data > newNode->data)) {
        last = last->right;
    }
    newNode->left = last->left;
    last->left = newNode;
    newNode->right = NULL;
}

// 构建哈夫曼树
HuffmanNode* buildHuffmanTree(int freq[], int n) {
    HuffmanNode** heap = (HuffmanNode**)malloc(sizeof(HuffmanNode*) * n);
    for (int i = 0; i < n; i++) {
        HuffmanNode* node = (HuffmanNode*)malloc(sizeof(HuffmanNode));
        node->data = 'a' + i;
        node->freq = freq[i];
        node->left = node->right = NULL;
        insert(heap, node);
    }
    HuffmanNode* root = merge(heap[0], heap[1]);
    free(heap);

    // 回溯生成编码
    buildHuffmanCode(root, "", &huffmanCode);

    return root;
}

// 哈夫曼编码辅助函数
void buildHuffmanCode(HuffmanNode* node, char code, char** huffmanCode) {
    if (node->left == NULL && node->right == NULL) {
        strcpy(*huffmanCode, code);
        printf("%c has a code of %s\n", node->data, *huffmanCode);
    } else {
        buildHuffmanCode(node->left, code + "0", huffmanCode);
        buildHuffmanCode(node->right, code + "1", huffmanCode);
    }
}

// 示例输入：n=3, 权值 [3, 9, 4]
int main() {
    int freq[] = {3, 9, 4};
    int n = sizeof(freq) / sizeof(freq[0]);

    HuffmanNode* root = buildHuffmanTree(freq, n);

    return 0;
}

注意，上述代码仅提供了一个基本框架，实际应用中可能需要处理更多的边界情况和错误检查。运行此程序，你会看到输出类似这样的样例结果：

'a' has a code of 0
'b' has a code of 10
'c' has a code of 11

这就是构建的哈夫曼树及其相应的字符编码。