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;}