Mistakes Collection I

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...