下面代码构建的树一定是完全二叉树：

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

TreeNode* buildCompleteBinaryTree() {
    TreeNode* root = new TreeNode{1};
    root->left = new TreeNode{2};
    root->right = new TreeNode{3};
    root->left->left = new TreeNode{4};
    root->left->right = new TreeNode{5};
    root->right->left = new TreeNode{6};
    return root;
}