Add merge-intervals solution

Author: Jeehay28

merge-intervals/Jeehay28.js

@@ -0,0 +1,34 @@
+// 🚀 My own approach!
+// ✅ Time Complexity: O(n log n), where n is the number of intervals
+// ✅ Space Complexity: O(n) (due to the stack storage)
+
+/**
+ * @param {number[][]} intervals
+ * @return {number[][]}
+ */
+var merge = function (intervals) {
+  intervals.sort((a, b) => a[0] - b[0]);
+  // Sorting takes O(n log n) time, where n is the number of intervals.
+  // JavaScript's sort() is O(1) (in-place) for sorting in most cases.
+
+  let start = intervals[0][0];
+  let end = intervals[0][1];
+  let stack = [intervals[0]]; // O(n) additional space usage.
+
+  for (const [s, e] of intervals) {
+    // This takes O(n) time.
+    const [prevStart, prevEnd] = stack[stack.length - 1];
+
+    if (prevStart <= s && s <= prevEnd) {
+      start = Math.min(s, start);
+      end = Math.max(e, end);
+      stack.pop();
+    } else {
+      start = s;
+      end = e;
+    }
+    stack.push([start, end]);
+  }
+  return stack;
+};
+