Edge-case checklists

You don't need every item every time. Before coding, ask about the ones that change your plan ("can it be empty?"). After coding, test the ones most likely to break your loops. Tick them as you practise, so they become a habit.

• Array

  • Empty array (length == 0).
  • One element, and two elements.
  • All elements the same.
  • Already sorted, and sorted in reverse.
  • Negatives and zero.
  • Duplicates.
  • The answer does not exist (target not present).
  • Large n, like 10⁵: will O(n²) be too slow?

  0 / 8 ticked

• String

  • Empty string "".
  • One character.
  • All the same character, like "aaaa".
  • Uppercase vs lowercase: does case matter? (ask)
  • Spaces, digits, punctuation: are they allowed? (ask)
  • Only a–z, or any character? It decides int[26] vs a HashMap.
  • Long strings: build with StringBuilder, not += in a loop.

  0 / 7 ticked

• Linked list

  • head == null.
  • One node, and two nodes.
  • Change at the head or the tail: a dummy node keeps it simple.
  • Even vs odd length (middle-node problems).
  • k larger than the list length.
  • Can there be a cycle? (ask)

  0 / 6 ticked

• Tree

  • root == null.
  • Only the root.
  • Skewed tree, like a linked list: recursion depth becomes n.
  • Negative values in nodes.
  • Duplicates in a BST: left, right, or not allowed? (ask)
  • The answer is at a leaf vs at the root.

  0 / 6 ticked

• Graph

  • One node, no edges.
  • Disconnected parts: loop over every node, not only node 0.
  • Cycles: keep a visited set.
  • Directed or undirected? (ask)
  • Self-loops or repeated edges? (ask)
  • Target not reachable: return -1, or what? (ask)

  0 / 6 ticked

• Matrix (grid)

  • Zero rows, or zero columns.
  • 1 × 1.
  • One row, or one column.
  • Not square: m rows × n columns, m ≠ n.
  • Neighbour checks stay inside the borders.
  • Change the grid in place, or keep the input safe? (ask)

  0 / 6 ticked

• Intervals

  • Empty list, and one interval.
  • Touching ends, like [1, 2] and [2, 3]: overlap or not? (ask)
  • One interval fully inside another.
  • Input not sorted: sort by start first.
  • Same start, different ends.

  0 / 5 ticked

These are Java rules, straight from the language spec. They catch even strong candidates.

• Overflow and sign traps

  • int stops at 2,147,483,647. Sums and products can wrap to a negative: use long.
  • Binary search middle: lo + (hi - lo) / 2, not (lo + hi) / 2.
  • % keeps the sign of the left number: -5 % 3 is -2, not 1.
  • Math.abs(Integer.MIN_VALUE) is still negative.
  • Dividing an int by zero throws ArithmeticException.
  • Compare Integer objects with .equals(), not ==.

  0 / 6 ticked

• Make edges a habit

  Every problem in this course lists its edge cases. Before you open them, guess them yourself.

  train itall problemsJava I/O & debugginglinked listsintervals