boolean function isAvl(tree t)
if leftChild(t)==null AND rightChild(t)==null return TRUE
int balance=relHeight(rightChild(t)) - relHeight(leftChild(t))
if balace>1 or balance<-1 return FALSE
else return isAvl(rightChild(t)) AND isAvl(leftChild(t))
end function
int function relHeight (tree t)
if t=NULL return 0
return 1  + max(relHeight(leftChild(t)), relHeight(rightChild(t)))
end function