对于递推式 f(a,b)=f(a−1,b)+f(a,b−1)f(a, b) = f(a-1, b) + f(a, b-1)f(a,b)=f(a−1,b)+f(a,b−1)，我们将其转化为组合数的形式，这对吗？
f(a,b)=f(a−1,b)+f(a,b−1)f(a, b) = f(a-1, b) + f(a, b-1)f(a,b)=f(a−1,b)+f(a,b−1)
=(a−1+bb)+(a+b−1a−1)= \binom{a-1+b}{b} + \binom{a+b-1}{a-1}=(ba−1+b)+(a−1a+b−1)
=(a+bb)= \binom{a+b}{b}=(ba+b)
