Stl pair code explanation

#1

Please provide the code how we will implement this whole code with STL library(Pair).
Thankyou

#2 
 pair<int,int> diameter(TreeNode* root)
{
    pair<int,int>p;
    if(root==NULL)
    {
        p.first=0;//height
        p.second=0;//diameter
        return p;
    }
    pair<int,int> left=diameter(root->left);
    pair<int,int> right=diameter(root->right);
    
    p.first=max(left.first,right.first)+1;
    p.second=max(left.first+right.first,max(left.second,right.second));
    return p;
}

The diameter function would be something like this

#3

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