Symbol
=>'s meaning:
what it used to be like => what it should be.
1) mistake array subscript: too small
a[n][2 * n] => a[2 * n][2 * n] => a[2 * n][2500]
2) mistake array visit
In this case, array is defined int tri[N][2].
tri[n][i] => tri[n][f]
3) mistake compare expression
if ('a' <= x <= 'z') do...
=> if ('a' <= x && x <= 'z') do...