TDOG集训define\(|s|:字符串s的长度\)\(fac_x:x的因数个数\)Day1T1.乘积累加和已完成Question：求C(n,0)*C(n,0)+C(n,1)*C(n,1)…+C(n,n)*C(n,n)，对\(10^9+7\)取模Analysis：数学原式可通过二项式定理转化成求\(C_{2n}^n\bmod1e9+7\)。Solution：预处理阶乘和逆元。剩