Add solution #587

Author: JoshCrozier

README.md

@@ -455,6 +455,7 @@
 576|[Out of Boundary Paths](./0576-out-of-boundary-paths.js)|Medium|
 581|[Shortest Unsorted Continuous Subarray](./0581-shortest-unsorted-continuous-subarray.js)|Medium|
 583|[Delete Operation for Two Strings](./0583-delete-operation-for-two-strings.js)|Medium|
+587|[Erect the Fence](./0587-erect-the-fence.js)|Hard|
 589|[N-ary Tree Preorder Traversal](./0589-n-ary-tree-preorder-traversal.js)|Easy|
 594|[Longest Harmonious Subsequence](./0594-longest-harmonious-subsequence.js)|Easy|
 599|[Minimum Index Sum of Two Lists](./0599-minimum-index-sum-of-two-lists.js)|Easy|

solutions/0587-erect-the-fence.js

@@ -0,0 +1,41 @@
+/**
+ * 587. Erect the Fence
+ * https://leetcode.com/problems/erect-the-fence/
+ * Difficulty: Hard
+ *
+ * You are given an array trees where trees[i] = [xi, yi] represents the location of
+ * a tree in the garden.
+ *
+ * Fence the entire garden using the minimum length of rope, as it is expensive. The
+ * garden is well-fenced only if all the trees are enclosed.
+ *
+ * Return the coordinates of trees that are exactly located on the fence perimeter.
+ * You may return the answer in any order.
+ */
+
+/**
+ * @param {number[][]} trees
+ * @return {number[][]}
+ */
+var outerTrees = function(trees) {
+  const a = [];
+  const b = [];
+
+  trees.sort(([x1, y1], [x2, y2]) => x1 === x2 ? y1 - y2 : x1 - x2);
+  for (const point of trees) {
+    while (a.length >= 2 && cross(a[a.length - 2], a[a.length - 1], point) < 0) {
+      a.pop();
+    }
+    a.push(point);
+    while (b.length >= 2 && cross(b[b.length - 2], b[b.length - 1], point) > 0) {
+      b.pop();
+    }
+    b.push(point);
+  }
+
+  return [...new Set([...a, ...b].map(p => JSON.stringify(p)))].map(p => JSON.parse(p));
+
+  function cross(p, q, r) {
+    return (q[0] - p[0]) * (r[1] - p[1]) - (q[1] - p[1]) * (r[0] - p[0]);
+  }
+};