P6475[NOIOnline#2入门组]建设城市传送门分类讨论：设\(f(x,y)\)为\(C^{j-1}_{i+j-1}\)\(x,y\)在同一旁把\(x,y\)之间的看成一个高楼公式\(f(n,m)\timesf(n+x-y,m)\)\(x,y\)在异侧枚举\(x,y\)高楼的高度\(h\)\(\displaystyle\sum^{n}_{i=1}f(x-1,i)*f(n-x,m-i