面试题19：如何判断一棵二叉树是完全二叉树？

参考：http://blog.csdn.net/njdragonfly/article/details/6373199

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() :
            val(0), left(nullptr), right(nullptr) {
    }
    TreeNode(int x) :
            val(x), left(nullptr), right(nullptr) {
    }
};
class Solution {
public:
    bool isCompleteTree(TreeNode* root){
        if(root == nullptr) return true;
        queue<TreeNode* > q;
        TreeNode* p = root;
        q.push(root);
        while(p){
            q.pop();
            q.push(p->left);
            q.push(p->right);
            p = q.front();
        }
        while(!q.empty()){
            p = q.front();
            q.pop();
            if(p != nullptr) return false;
        }
        return true;
    }
};