  Recently, Han obtained two hook length formulas for binary trees and asked
for combinatorial proofs. One of Han's formulas has been generalized to k-ary
trees by Yang. Sagan has found a probabilistic proof of Yang's extension. We
give combinatorial proofs of Yang's formula for k-ary trees and the other
formula of Han for binary trees. Our bijections are based on the structure of
k-ary trees with staircase labelings.
