958. Check Completeness of a Binary Tree

class Solution {
public:
    bool isCompleteTree(TreeNode* root) {
        if(root==NULL){
            return true;
        }
        queue<TreeNode*> q;
        q.push(root);
        bool end=false;
        while(!q.empty()){
            TreeNode* leftnode=q.front();
            q.pop();
            if(!leftnode){
                end=true;
            }else{
                if(end){
                    return false;
                }
                q.push(leftnode->left);
                q.push(leftnode->right);
            }
        }
        return true;
    }
};