How it works and how it give the answer 3?

Question

int mat[2][2] = {{1,2},{3,4}};
cout<<0[1[mat]];

Answer 1

Suppose `arr` is an array and `n` is a interger then `n[arr]` is same as `arr[n]`.

Now adding one more dimension `m` to array; `m[n[arr]]` is same as arr[n][m]

in your code `0[1[mat]]` means `mat[1][0]`