pku2346 Lucky tickets

准确的说，这个题叫递推，不叫DP，

用f[i,j]表示长度为i，和为j的方案数

f[i,j]=∑f[i-1,j-k]

边界见程序。

program pku2346(input,output);
var
    f:array[0..5,0..45] of int64;
    answer:array[0..11] of int64;
    n:longint;
    i,j,k:longint;
begin
    fillchar(f,sizeof(f),0);
    for i:=0 to 9 do
        f[1,i]:=1;
    for i:=1 to 5 do
        for j:=0 to 45 do
            if f[i-1,j]>0 then
            for k:=0 to 9 do
                if j+k<=45 then
                    f[i,j+k]:=f[i,j+k]+f[i-1,j];
    answer[0]:=0;
    for i:=1 to 5 do
    begin    
        answer[i]:=0;
        for j:=0 to 45 do
            inc(answer[i],f[i,j]*f[i,j]);
    end;
    readln(n);
    writeln(answer[n>>1]);
end.