关于$visit$$(T2)$$O(1)$式子推导

$\sum_{k=0}^{t-n-m} C_{t}^{n+k}\times C_{n+k}^{k/2} \times C_{t-n-k}^{(t-n-m-k)/2}$

$=\sum_{k=0}^{t-n-m}\frac{t!}{(n+k)!\times (t-n-k)!}\times\frac{(n+k)!}{(n+k/2)!\times(k/2)!}\times \frac{(t-n-k)!}{((t-n-k-m)/2)!\times{((t-n-k+m)/2)!}}$

$=C_{t}^{(t+m+n)/2}\times\sum_{k=0}^{t-n-m}C_{(t+m+n)/2}^{n+k/2}\times C_{(t-n-m)/2}^{(t-n-m-k)/2}$

$=C_{t}^{(t+n+m)/2}\times C_{t}^{(t+n-m)/2}$