Add solution #218

Author: JoshCrozier

README.md

@@ -199,6 +199,7 @@
 215|[Kth Largest Element in an Array](./0215-kth-largest-element-in-an-array.js)|Medium|
 216|[Combination Sum III](./0216-combination-sum-iii.js)|Medium|
 217|[Contains Duplicate](./0217-contains-duplicate.js)|Easy|
+218|[The Skyline Problem](./0218-the-skyline-problem.js)|Hard|
 219|[Contains Duplicate II](./0219-contains-duplicate-ii.js)|Easy|
 220|[Contains Duplicate III](./0220-contains-duplicate-iii.js)|Hard|
 222|[Count Complete Tree Nodes](./0222-count-complete-tree-nodes.js)|Easy|

solutions/0218-the-skyline-problem.js

@@ -0,0 +1,54 @@
+/**
+ * 218. The Skyline Problem
+ * https://leetcode.com/problems/the-skyline-problem/
+ * Difficulty: Hard
+ *
+ * A city's skyline is the outer contour of the silhouette formed by all the buildings in that
+ * city when viewed from a distance. Given the locations and heights of all the buildings,
+ * return the skyline formed by these buildings collectively.
+ *
+ * The geometric information of each building is given in the array buildings where
+ * buildings[i] = [lefti, righti, heighti]:
+ * - lefti is the x coordinate of the left edge of the ith building.
+ * - righti is the x coordinate of the right edge of the ith building.
+ * - heighti is the height of the ith building.
+ *
+ * You may assume all buildings are perfect rectangles grounded on an absolutely flat surface
+ * at height 0.
+ *
+ * The skyline should be represented as a list of "key points" sorted by their x-coordinate
+ * in the form [[x1,y1],[x2,y2],...]. Each key point is the left endpoint of some horizontal
+ * segment in the skyline except the last point in the list, which always has a y-coordinate
+ * 0 and is used to mark the skyline's termination where the rightmost building ends. Any ground
+ * between the leftmost and rightmost buildings should be part of the skyline's contour.
+ *
+ * Note: There must be no consecutive horizontal lines of equal height in the output skyline.
+ * For instance, [...,[2 3],[4 5],[7 5],[11 5],[12 7],...] is not acceptable; the three lines
+ * of height 5 should be merged into one in the final output as such: [...,[2 3],[4 5],[12 7],...]
+ */
+
+/**
+ * @param {number[][]} buildings
+ * @return {number[][]}
+ */
+var getSkyline = function(buildings) {
+  const result = [];
+  const events = buildings.flatMap(([l, r, h]) => [[l, -h], [r, h]]);
+
+  events.sort((a, b) => a[0] - b[0] || a[1] - b[1]);
+
+  for (let i = 0, previous = 0, heights = [0]; i < events.length; i++) {
+    const [x, h] = events[i];
+    if (h < 0) {
+      heights.push(-h);
+    } else {
+      heights.splice(heights.indexOf(h), 1);
+    }
+    const value = Math.max(...heights);
+    if (value !== previous) {
+      result.push([x, previous = value]);
+    }
+  }
+
+  return result;
+};