Check Completeness of a Binary Tree_2.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isCompleteTree(TreeNode* root) {
        if(!root)return true;
        queue<TreeNode*> q;
        int depth = 0;
        q.push(root);
        while(q.size())
        {
            int len = q.size();
            bool hasNull = false;
            if((1 << depth) != len)
            {
                //verify if it is the last laryer
                for(int i = 0; i < len; ++i)
                {
                    auto node = q.front();
                    q.pop();
                    if(node->left || node->right)return false;
                }
            }
            else
            {
                //verify all nodes in next level are posed as left as possible
                for(int i = 0; i < len; ++i)
                {
                    auto node = q.front();
                    q.pop();
                    if(node->left){ if(hasNull)return false; q.push(node->left); }
                    else hasNull = true;
                    if(node->right){ if(hasNull)return false; q.push(node->right); }
                    else hasNull = true;
                }
            }
            ++depth;
        }
        return true;
    }
};