面试题19：如何判断一棵二叉树是完全二叉树？

1 struct TreeNode {
2     int val;
3     TreeNode* left;
4     TreeNode* right;
5     TreeNode() :
6             val(0), left(nullptr), right(nullptr) {
7     }
8     TreeNode(int x) :
9             val(x), left(nullptr), right(nullptr) {
10     }
11 };
12 class Solution {
13 public:
14     bool isCompleteTree(TreeNode* root){
15         if(root == nullptr) return true;
16         queue<TreeNode* > q;
17         TreeNode* p = root;
18         q.push(root);
19         while(p){
20             q.pop();
21             q.push(p->left);
22             q.push(p->right);
23             p = q.front();
24         }
25         while(!q.empty()){
26             p = q.front();
27             q.pop();
28             if(p != nullptr) return false;
29         }
30         return true;
31     }
32 };