两个指针,i,j;
F(i,j)为将S字符串的i到j位变为合法的代价
读入从0开始

For i m-1 to 1 
    For j i+1 to m-1 
    {
        F(i,j)=min(  F(i+1,j)+cost[i]  ,  F(i,j-1)+cost[j]  );
        IF(S[i]==S[j])
            F(i,j)=min(  F(i,j)  ,  F(i+1,j-1) )
    }