Add solution #980

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,062 LeetCode solutions in JavaScript
+# 1,063 LeetCode solutions in JavaScript
 
 [https://leetcode.com/](https://leetcode.com/)
 
@@ -788,6 +788,7 @@
 977|[Squares of a Sorted Array](./solutions/0977-squares-of-a-sorted-array.js)|Easy|
 978|[Longest Turbulent Subarray](./solutions/0978-longest-turbulent-subarray.js)|Medium|
 979|[Distribute Coins in Binary Tree](./solutions/0979-distribute-coins-in-binary-tree.js)|Medium|
+980|[Unique Paths III](./solutions/0980-unique-paths-iii.js)|Hard|
 985|[Sum of Even Numbers After Queries](./solutions/0985-sum-of-even-numbers-after-queries.js)|Easy|
 989|[Add to Array-Form of Integer](./solutions/0989-add-to-array-form-of-integer.js)|Easy|
 994|[Rotting Oranges](./solutions/0994-rotting-oranges.js)|Medium|

solutions/0980-unique-paths-iii.js

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+/**
+ * 980. Unique Paths III
+ * https://leetcode.com/problems/unique-paths-iii/
+ * Difficulty: Hard
+ *
+ * You are given an m x n integer array grid where grid[i][j] could be:
+ * - 1 representing the starting square. There is exactly one starting square.
+ * - 2 representing the ending square. There is exactly one ending square.
+ * - 0 representing empty squares we can walk over.
+ * - -1 representing obstacles that we cannot walk over.
+ *
+ * Return the number of 4-directional walks from the starting square to the ending
+ * square, that walk over every non-obstacle square exactly once.
+ */
+
+/**
+ * @param {number[][]} grid
+ * @return {number}
+ */
+var uniquePathsIII = function(grid) {
+  const rows = grid.length;
+  const cols = grid[0].length;
+  let emptySquares = 0;
+  let startRow;
+  let startCol;
+
+  for (let i = 0; i < rows; i++) {
+    for (let j = 0; j < cols; j++) {
+      if (grid[i][j] === 0) emptySquares++;
+      if (grid[i][j] === 1) [startRow, startCol] = [i, j];
+    }
+  }
+
+  function explorePaths(row, col, remaining) {
+    if (row < 0 || row >= rows || col < 0 || col >= cols || grid[row][col] < 0) {
+      return 0;
+    }
+    if (grid[row][col] === 2) {
+      return remaining === 0 ? 1 : 0;
+    }
+
+    const current = grid[row][col];
+    grid[row][col] = -1;
+    const directions = [[0, 1], [1, 0], [0, -1], [-1, 0]];
+    let pathCount = 0;
+
+    for (const [dr, dc] of directions) {
+      pathCount += explorePaths(row + dr, col + dc, remaining - 1);
+    }
+
+    grid[row][col] = current;
+    return pathCount;
+  }
+
+  return explorePaths(startRow, startCol, emptySquares + 1);
+};