Futoshiki

Fill every square with 1–4 on a 4 × 4 board, or 1–5 on a 5 × 5 board. Use each number once in every row and column. Obey every inequality: its pointed end faces the smaller number. There are no smaller Sudoku boxes.

Tap a square, then a number below the board. Underlined numbers are fixed clues. Erase clears the selected number and its notes. Turn Notes on to toggle possible numbers in a blank square. A selected square has an inset outline; rule conflicts stay visible until changed or undone.

Keyboard: Tab to the board; arrows move, Home/End move along the row. Type a digit to enter it; N toggles Notes. Delete or Backspace clears the selected square. Ctrl/Cmd+Z undoes; Ctrl/Cmd+Shift+Z or Ctrl/Cmd+Y redoes. The number buttons and all other controls work with Enter or Space.

This finite collection has 12 original puzzles at each size. Size is not a difficulty rating. Next chooses another puzzle at your size; changing size or restarting asks before replacing unfinished choices. Local resume stays in this browser, without accounts or sync. Undo history lasts for this session.

Start with long inequality chains: a chain of four increasing cells on a 4 × 4 board must be 1, 2, 3, 4. A number already in a row or column cannot appear there again.

Hint reasons from your current numbers and notes. If a square has notes, only those candidates are allowed during hint checking; an empty note list leaves all numbers possible. Notes are not filled in or corrected automatically. If a hint reports a contradiction, change a number, clear restrictive notes or undo.

Hints explain constraint elimination or a value proved by bounded search. A search limit means no move was proved within the budget. Apply hint is a separate action. Hints and reveals mark the board as assisted; a reveal replaces all choices after confirmation.

Rules reference: Puzzler’s Futoshiki instructions. All boards, code, diagrams and explanations here are original.