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vector<int> zigzagTreeTraversal(TreeNode<int> *root)
{
    vector<int> result;   //net ans
    if(root==NULL) return result; 
    bool lefttoright=true;
    queue<TreeNode<int>*> q;
    q.push(root);

    while(!q.empty()){
    
        int size=q.size();
        vector<int> ans(size); //for each level ans
        
        for(int i=0;i<size;i++){
            TreeNode<int>*front=q.front();
            q.pop();
            int index= lefttoright?i :size-i-1;
            ans[index]=front->data;
            if(front->left) q.push(front->left);  //later levels
            if(front->right) q.push(front->right);
        }
        
        lefttoright=!lefttoright;
        for(auto i:ans) result.push_back(i);
    }

    return result;
}