Given two binary trees, write a function to check if they are equal or not.
Two binary trees are considered equal if they are structurally identical and the nodes have the same value.
if(p==q) return true; if(p==null && q==null) return true; if(p==null || q==null) { return false; } if(p.val!=q.val) return false; return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);