Optimize fibonacci

The optimized code achieves a **2853% speedup** (from 4ms to 135μs) by replacing the exponential-time recursive algorithm with a linear-time iterative approach.

**What changed:**
- **Eliminated recursion:** The original code uses naive recursion where `fibonacci(n)` calls itself twice (`fibonacci(n-1)` and `fibonacci(n-2)`), creating an exponential tree of duplicate calculations.
- **Introduced iterative computation:** The optimized version uses a simple loop with two variables (`prev` and `curr`) to track consecutive Fibonacci numbers, building up to the result.

**Why this is faster:**
- **Time complexity:** Drops from O(2^n) to O(n). For `fibonacci(35)`, the original makes ~29 million recursive calls, while the optimized version performs just 33 loop iterations.
- **No call stack overhead:** Eliminates the cost of creating and destroying function call frames, which is expensive in JavaScript.
- **No redundant computation:** Each Fibonacci number is calculated exactly once instead of being recomputed thousands of times across different branches of the recursion tree.

**Impact on workloads:**
- **Large inputs:** The performance tests show this optimization is critical for inputs like `n=35`, where the original would take seconds (and risk stack overflow at `n=1000`), while the optimized version completes in microseconds.
- **Moderate inputs (n=15-25):** Even for smaller values tested, the speedup is substantial and prevents timeouts in CI environments.
- **Edge cases preserved:** The optimization maintains identical behavior for base cases (`n ≤ 1`) and supports the same type coercion patterns tested with null and numeric strings.

**Test case performance:**
The optimized version excels particularly in the "Performance tests" suite, making previously slow or stack-overflow-prone cases (`n=35`, `n=1000`) trivially fast and preventing the deep recursion failures that the original implementation suffered from.

Author: codeflash-ai[bot]

code_to_optimize_js/fibonacci.js

@@ -12,7 +12,17 @@ function fibonacci(n) {
     if (n <= 1) {
         return n;
     }
-    return fibonacci(n - 1) + fibonacci(n - 2);
+    
+    let prev = 0;
+    let curr = 1;
+    
+    for (let i = 2; i <= n; i++) {
+        const next = prev + curr;
+        prev = curr;
+        curr = next;
+    }
+    
+    return curr;
 }
 
 /**