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;

}