习题6.22 习题6.23 习题6.24 习题6.31

//6.31
void post2pre(Xtype post[],Xtype pre[],int postart ,int poend ,int pestart,int peend )
{
int half ;
if (poend >= postart)
{
pre[pestart] = post[poend] ;
half = (poend - postart) /2 ;
post2pre(post , pre , postart                      , postart + half - 1 , pestart + 1        , pestart + half  ) ;
post2pre(post , pre , postart + half               , poend - 1          , pestart + half + 1     , peend           ) ;
}
}