c语言输出二叉树先序遍历结果的第k个结点

以下是输出二叉树先序遍历结果的第k个结点的C语言代码：

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

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

void preOrder(struct TreeNode* root, int* count, int k, struct TreeNode** result) {
    if (root == NULL) {
        return;
    }
    (*count)++;
    if (*count == k) {
        *result = root;
        return;
    }
    preOrder(root->left, count, k, result);
    preOrder(root->right, count, k, result);
}

struct TreeNode* getKthNode(struct TreeNode* root, int k) {
    if (root == NULL) {
        return NULL;
    }
    int count = 0;
    struct TreeNode* result = NULL;
    preOrder(root, &count, k, &result);
    return result;
}

int main() {
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val = 1;
    root->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->left->val = 2;
    root->left->left = NULL;
    root->left->right = NULL;
    root->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->val = 3;
    root->right->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->left->val = 4;
    root->right->left->left = NULL;
    root->right->left->right = NULL;
    root->right->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->right->right->val = 5;
    root->right->right->left = NULL;
    root->right->right->right = NULL;
    int k = 4;
    struct TreeNode* result = getKthNode(root, k);
    if (result != NULL) {
        printf("The %dth node of the binary tree is %d.\n", k, result->val);
    } else {
        printf("The binary tree doesn't have %d nodes.\n", k);
    }
    return 0;
}

在这个例子中，我们首先定义了一个二叉树结构体`struct TreeNode`，其中包含一个整型变量`val`表示结点的值，以及两个指向左右子结点的指针`left`和`right`。我们接着定义了一个函数`preOrder`，用于实现先序遍历，并通过参数`count`记录已经遍历的结点个数，若已经遍历的结点个数等于目标结点的序号$k$，则将该结点的指针存入`result`中。最终，我们定义了一个函数`getKthNode`，用于调用`preOrder`函数获取二叉树先序遍历结果的第$k$个结点。最后，我们通过一个例子进行测试，输出二叉树先序遍历结果的第$k$个结点的值。