Tic-tac-toe II

Generalize tic-tac-toe to an N×N board where M consecutive marks win. The fixed 3×3 board with eight hard-coded lines doesn't scale — for a 9×9 board you can't list every line by hand. Instead, store the board as a flat array and detect a win by scanning from each filled cell along four directions for M in a row.

Signature

type Mark = 'X' | 'O' | null;
function getWinner(board: Mark[], size: number, target: number): Mark;

// A self-contained component. No props.
function App(): JSX.Element;

Examples

size = 5, target = 4 → place 4 X's in a row/col/diagonal to win
size = 3, target = 3 → classic tic-tac-toe

cell at row r, column c lives at flat index r * size + c
scan directions: → (0,1), ↓ (1,0), ↘ (1,1), ↙ (1,-1)

Notes

• Flat board + index math. board[r * size + c]; don't hard-code line lists.
• Scan, don't enumerate. From each filled cell, step up to M-1 along 4 directions; M in a row wins.
• Parameterized. size and target come from controls; changing either resets the board.
• Out of scope. Unbeatable AI, gravity (that's Connect Four) — two human players here.