Question

why nodes with 2 children have degree 3 in a binary tree?

Answer 1

Degree of a node - No of edges connected to the node.

node with 2 children has degree 3.( except root which has degree = 2).
node with 1 children has degree 2.( except root which has degree = 1).
leaf node has degree = 1.

Answer 2

Degree of a node in binary tree is the number of subtrees it have or we can say no .of childrens a node have,

a node with two chidren have 2 degree it can not have 3 degree...maximum degree of a binary tree is 2..

leaf node have 0 degree