Add solution #794

Author: JoshCrozier

README.md

@@ -604,6 +604,7 @@
 791|[Custom Sort String](./0791-custom-sort-string.js)|Medium|
 792|[Number of Matching Subsequences](./0792-number-of-matching-subsequences.js)|Medium|
 793|[Preimage Size of Factorial Zeroes Function](./0793-preimage-size-of-factorial-zeroes-function.js)|Hard|
+794|[Valid Tic-Tac-Toe State](./0794-valid-tic-tac-toe-state.js)|Medium|
 796|[Rotate String](./0796-rotate-string.js)|Easy|
 802|[Find Eventual Safe States](./0802-find-eventual-safe-states.js)|Medium|
 804|[Unique Morse Code Words](./0804-unique-morse-code-words.js)|Easy|

solutions/0794-valid-tic-tac-toe-state.js

@@ -0,0 +1,58 @@
+/**
+ * 794. Valid Tic-Tac-Toe State
+ * https://leetcode.com/problems/valid-tic-tac-toe-state/
+ * Difficulty: Medium
+ *
+ * Given a Tic-Tac-Toe board as a string array board, return true if and only if it is possible
+ * to reach this board position during the course of a valid tic-tac-toe game.
+ *
+ * The board is a 3 x 3 array that consists of characters ' ', 'X', and 'O'. The ' ' character
+ * represents an empty square.
+ *
+ * Here are the rules of Tic-Tac-Toe:
+ * - Players take turns placing characters into empty squares ' '.
+ * - The first player always places 'X' characters, while the second player always places 'O'
+ *   characters.
+ * - 'X' and 'O' characters are always placed into empty squares, never filled ones.
+ * - The game ends when there are three of the same (non-empty) character filling any row,
+ *   column, or diagonal.
+ * - The game also ends if all squares are non-empty.
+ * - No more moves can be played if the game is over.
+ */
+
+/**
+ * @param {string[]} board
+ * @return {boolean}
+ */
+var validTicTacToe = function(board) {
+  let xCount = 0;
+  let oCount = 0;
+
+  for (const row of board) {
+    for (const cell of row) {
+      if (cell === 'X') xCount++;
+      if (cell === 'O') oCount++;
+    }
+  }
+
+  if (xCount !== oCount && xCount !== oCount + 1) return false;
+
+  const hasWon = player => {
+    for (let i = 0; i < 3; i++) {
+      if (board[i] === player.repeat(3)) return true;
+      if (board[0][i] === player && board[1][i] === player && board[2][i] === player) return true;
+    }
+
+    return (board[0][0] === player && board[1][1] === player && board[2][2] === player)
+      || (board[0][2] === player && board[1][1] === player && board[2][0] === player);
+  };
+
+  const xWin = hasWon('X');
+  const oWin = hasWon('O');
+
+  if (xWin && xCount !== oCount + 1) return false;
+  if (oWin && xCount !== oCount) return false;
+  if (xWin && oWin) return false;
+
+  return true;
+};