编写C语言程序，将一棵二叉树中所有结点的左、右子女

的值交换。

以下是C语言代码实现：

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

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

void swapChild(struct TreeNode* root) {
    if(root == NULL) return;
    struct TreeNode* temp = root->left;
    root->left = root->right;
    root->right = temp;
    swapChild(root->left);
    swapChild(root->right);
}

int main() {
    // 假设二叉树已经存在并且被初始化
    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 = NULL;
    root->right->right = NULL;

    swapChild(root);

    // 输出交换后的二叉树结构
    printf("root: %d\n", root->val);
    printf("root->left: %d\n", root->left->val);
    printf("root->right: %d\n", root->right->val);
    return 0;
}

运行结果为：

root: 1
root->left: 3
root->right: 2

可以看到，左、右子女的值已经交换成功。