Answer: A very commonly seen shrub with bright yellow flowers in early spring is of course the forsythia.
But in the story book called "A Tree Grows In Brooklyn" the tree is (as I remember being told long ago in a horticulture class) Ailanthus altissima, sometimes called "Tree of Heaven". This tree blooms in June and has 8 to 16 inch long greenish-yellow blooms that hang down; another identifying characteristic is that on male trees the flowers stink and on female trees they are odorless. Maybe this is what you are thinking of.
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