Linked lists are not suitable data structures of which one of the following problems?

Which of the following permutations can be obtained in the output (in the same order) using a stack assuming that the input is the sequence 1, 2, 3, 4, 5 in that order?

In a compact single dimensional array representation for lower triangular matrices (i.e all the elements above the diagonal are zero) of size n $$\times$$ n, non-zero elements (i.e., elements of the lower triangle) of each row are stored one after another, starting from the first row, the index of the (i, j)^th element of the lower triangular matrix in this new representation is

Give a relational algebra expression using only the minimum number of operators from $$\left( { \cup ,\, - } \right)$$ which is equivalent to $$R \cap S$$.