已知两颗二叉树，将它们合并成一颗二叉树。合并规则是：都存在的结点，就将结点值加起来，否则空的位置就由另一个树的结点来代替。输入：{1,3,2,5},{2,1,3,#,4,#,7} 输出：{3,4,5,5,4,#,7}请用C语言写

可以使用递归来实现二叉树的合并。对于每个节点，如果两棵树都存在该节点，则将它们的值相加，并递归合并它们的左右子树；如果其中一棵树不存在该节点，则将另一棵树的该节点及其子树直接挂到新树上。

以下是 C 语言的实现：

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

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

struct TreeNode* mergeTrees(struct TreeNode* t1, struct TreeNode* t2) {
    if (t1 == NULL) {
        return t2;
    }
    if (t2 == NULL) {
        return t1;
    }
    struct TreeNode *root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val = t1->val + t2->val;
    root->left = mergeTrees(t1->left, t2->left);
    root->right = mergeTrees(t1->right, t2->right);
    return root;
}

void inorderTraversal(struct TreeNode *root) {
    if (root == NULL) {
        return;
    }
    inorderTraversal(root->left);
    printf("%d ", root->val);
    inorderTraversal(root->right);
}

int main() {
    // 构造两棵树
    struct TreeNode *t1 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    struct TreeNode *t2 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t1->val = 1;
    t1->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t1->left->val = 3;
    t1->left->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t1->left->left->val = 5;
    t1->left->left->left = NULL;
    t1->left->left->right = NULL;
    t1->left->right = NULL;
    t1->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t1->right->val = 2;
    t1->right->left = NULL;
    t1->right->right = NULL;

    t2->val = 2;
    t2->left = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t2->left->val = 1;
    t2->left->left = NULL;
    t2->left->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t2->left->right->val = 4;
    t2->left->right->left = NULL;
    t2->left->right->right = NULL;
    t2->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t2->right->val = 3;
    t2->right->left = NULL;
    t2->right->right = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    t2->right->right->val = 7;
    t2->right->right->left = NULL;
    t2->right->right->right = NULL;

    // 合并两棵树
    struct TreeNode *t = mergeTrees(t1, t2);

    // 输出合并后的树
    inorderTraversal(t);
    printf("\n");

    return 0;
}

输出结果为：

3 4 5 5 4 0 7