C语言用设计算法判断结点E是否为结点E1和E2的共同祖先。

以下是C语言的算法，用于判断结点E是否为结点E1和E2的共同祖先：

// 定义二叉树结构体
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
};

// 判断结点E是否为结点E1和E2的共同祖先
struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {
    if (root == NULL || root == p || root == q) {
        return root;
    }
    struct TreeNode* left = lowestCommonAncestor(root->left, p, q);
    struct TreeNode* right = lowestCommonAncestor(root->right, p, q);
    if (left == NULL) {
        return right;
    } else if (right == NULL) {
        return left;
    } else {
        return root;
    }
}