First, We will store 1 as primary value of Subtree size for Each node . Then Whenever we find a child of a Node we add child's subtree size to the Parent's Subtree size . That means, Suppose we have node U and it's Subtree size is X . One of it's Child is node V and it's Subtree size is Y . Then we will add this Y to X .So subtree size of node U will be X+Y . This way we can calculate Subtree size for every Node . Because Child's Descendant is also a Descendant of Parent . That Means if a Node P is a part of node V's Subtree and Node V is child of node U then Node P is also a part of node U's subtree .