It can be done with only one Queue, doing a level traversal and each time we are in a new level we insert a dummy node(can be null)to separate the levels, while doing the traversal we just keep adding the nodes we are inserting and keep the count of how many nodes we have so when we find a null pointer we just divide the current sum by the current count.

vector TreeAverage(TreeNode* root) { vector result; if(root == NULL) return result; queue<div>q; q.push_back(root); while(!q.empty()) { int size = q.size(); int temp; int sum = 0; while(temp) { TreeNode* currNode = q.front(); q.pop(); sum = sum + q->val; if(q->left) { q.push(q->left); } if(q->right) { q.push(q->right); } temp--; } result.push_back(sum / size); } return result; }</div>

Update to previous submission : queue<div>q; int temp = size;</div>

use two queues, one for current level, one for level below, then it's just iterating over a queue