feat: Add deep-merge nested objects exercise with circular reference handling

Author: Pasupuleti-Veda-Vyasa

coding-exercise/deep-merge-nested-objects.md

@@ -0,0 +1,49 @@
+# Exercise: Deep-Merge Two Nested Objects With Circular References
+
+### Problem Statement
+
+Write a function **`deepMerge(objA, objB)`** that merges two JavaScript objects **without mutating** either of the original objects.
+
+The function must follow these rules:
+
+1. If a property exists in only one object, take that value.
+2. If the property exists in both objects:
+   - If both values are **plain objects**, recursively merge them.
+   - If both values are **arrays**, concatenate them.
+   - Otherwise, the value from **objB** overwrites the one from **objA**.
+3. Handle **circular references** safely.  
+   If you encounter the same reference path again, avoid infinite recursion.
+4. Ensure the merge is **immutable** — return a **new** object.
+5. Ensure the solution runs in **O(n)** time where *n* is the total number of properties.
+
+---
+
+### Example:
+
+```js
+const a = { 
+    x: { y: 1 },
+    arr: [1, 2],
+};
+a.self = a; // Circular reference
+
+const b = { 
+    x: { z: 2 },
+    arr: [3],
+    extra: true 
+};
+b.loop = b; // Circular reference
+
+const result = deepMerge(a, b);
+
+/*
+Expected structure:
+
+{
+  x: { y: 1, z: 2 },
+  arr: [1, 2, 3],
+  extra: true,
+  self: <circular ref>,
+  loop: <circular ref>
+}
+*/