feat: Solve 2d spiral array

  - Facebook swe prep.

Author: brianr482

2d-spiral.js

@@ -0,0 +1,81 @@
+
+/**
+ * Facebook, prep: Question 1 - 2D Spiral array
+ * Find the pattern and complete the function: 
+ * int[][] spiral(int n);
+ * where n is the size of the 2D array.
+*/
+/**
+ * @param {number} n Size of the array
+ * @return {number[][]} Array populated in spiral way
+ */
+const spiral = n => {
+  // Check edge case
+  if (n <= 0) {
+    return [];
+  }
+  
+  // 0 horizontally, 1 vertically
+  let axis = 0;
+  // 0 right/down, 1 left/up
+  let dir = 0
+  const area = n * n;
+  let i = 0;
+  let j = 0;
+  let counter = 1;
+  const arr = Array(n).fill().map(() => Array(n).fill());
+  arr[i][j] = counter;
+  counter++;
+  bounds = [0, n - 1];
+  // O(n^2) because the area is n * n and we loop the entire area
+  while (counter <= area) {
+    // Move in the current axis according to the current direction
+    if (axis === 0) {
+      j = dir === 0 ? j + 1 : j - 1;
+    } else {
+      i = dir === 0 ? i + 1 : i - 1;
+    }
+    /* 
+     * Check if the loop returned to the start.
+     * In that case, narrow the boundaries
+    */
+    if (arr[i][j]) {
+     bounds[0]++;
+     bounds[1]--;
+     i = bounds[0];
+     j = bounds[0];
+     arr[i][j] = counter;
+     axis = 0;
+     dir = 0;
+    } else {
+      arr[i][j] = counter;
+      /* 
+       * Check if the loop is at the limit of Y axis.
+       * In that case, switch the direction and the axis
+      */
+      if ((i === bounds[1] || i === bounds[0]) && axis === 1) {
+        axis = 0;
+        dir = i === 0 ? 0 : 1;
+       
+      /* 
+       * Check if the loop is at the limit of X axis.
+       * In that case, switch the axis
+      */
+      } else if ((j === bounds[1] || j === bounds[0]) && axis === 0) {
+        axis = 1;
+      }
+    }
+    counter++;
+  }
+  return arr;
+};
+
+const n = 4;
+const a = spiral(n);
+for (row of a) {
+  console.log(
+    row.map(el =>
+      el <= 9 ? `${'0'.repeat((n * n + '').length - 1)}${el}` : el.toString()
+    )
+  );
+}