Add solution #909

Author: JoshCrozier

README.md

@@ -484,6 +484,7 @@
 890|[Find and Replace Pattern](./0890-find-and-replace-pattern.js)|Medium|
 901|[Online Stock Span](./0901-online-stock-span.js)|Medium|
 905|[Sort Array By Parity](./0905-sort-array-by-parity.js)|Easy|
+909|[Snakes and Ladders](./0909-snakes-and-ladders.js)|Medium|
 912|[Sort an Array](./0912-sort-an-array.js)|Medium|
 914|[X of a Kind in a Deck of Cards](./0914-x-of-a-kind-in-a-deck-of-cards.js)|Medium|
 916|[Word Subsets](./0916-word-subsets.js)|Medium|

solutions/0909-snakes-and-ladders.js

@@ -0,0 +1,70 @@
+/**
+ * 909. Snakes and Ladders
+ * https://leetcode.com/problems/snakes-and-ladders/
+ * Difficulty: Medium
+ *
+ * You are given an n x n integer matrix board where the cells are labeled from 1 to n2 in a
+ * Boustrophedon style starting from the bottom left of the board (i.e. board[n - 1][0]) and
+ * alternating direction each row.
+ *
+ * You start on square 1 of the board. In each move, starting from square curr, do the following:
+ * - Choose a destination square next with a label in the range [curr + 1, min(curr + 6, n2)].
+ *   - This choice simulates the result of a standard 6-sided die roll: i.e., there are always at
+ *     most 6 destinations, regardless of the size of the board.
+ * - If next has a snake or ladder, you must move to the destination of that snake or ladder.
+ *   Otherwise, you move to next.
+ * - The game ends when you reach the square n2.
+ *
+ * A board square on row r and column c has a snake or ladder if board[r][c] != -1. The destination
+ * of that snake or ladder is board[r][c]. Squares 1 and n2 are not the starting points of any
+ * snake or ladder.
+ *
+ * Note that you only take a snake or ladder at most once per dice roll. If the destination to a
+ * snake or ladder is the start of another snake or ladder, you do not follow the subsequent snake
+ * or ladder.
+ *
+ * - For example, suppose the board is [[-1,4],[-1,3]], and on the first move, your destination
+ *   square is 2. You follow the ladder to square 3, but do not follow the subsequent ladder to 4.
+ *
+ * Return the least number of dice rolls required to reach the square n2. If it is not possible to
+ * reach the square, return -1.
+ */
+
+/**
+ * @param {number[][]} board
+ * @return {number}
+ */
+var snakesAndLadders = function(board) {
+  const target = board.length * board.length;
+  const queue = [1];
+  const seen = new Set([1]);
+  let moves = 0;
+
+  while (queue.length) {
+    const size = queue.length;
+    for (let i = 0; i < size; i++) {
+      const current = queue.shift();
+      if (current === target) return moves;
+      for (let dice = 1; dice <= 6 && current + dice <= target; dice++) {
+        const next = current + dice;
+        const [r, c] = getCoordinates(next);
+        const destination = board[r][c] === -1 ? next : board[r][c];
+        if (!seen.has(destination)) {
+          seen.add(destination);
+          queue.push(destination);
+        }
+      }
+    }
+    moves++;
+  }
+
+  return -1;
+
+  function getCoordinates(square) {
+    const r = board.length - 1 - Math.floor((square - 1) / board.length);
+    const c = (board.length - 1 - r) % 2 === 0
+      ? (square - 1) % board.length
+      : board.length - 1 - (square - 1) % board.length;
+    return [r, c];
+  }
+};