N1+2片叶子.
设有x片叶子,则此树有N1+N2+x个节点,树的边数比节点数少1,是N1+N2+x-1条边,由握手定理,3×N1+2×N2+x×1=2(N1+N2+x-1),解得x=N1+2,所以有N1+2片叶子.