find least common ancestor of a binary tree

lca(a, p, q)

if(!a)
return 0

if(a == p || a == q)
return a

l = lca(a->left, p, q)
r = lca(a->right, p, q)

if(l && r)
return a

if(l)
return l

if(r)
return r

return 0