diff --git a/rounds/1_histogram/solution.py b/rounds/1_histogram/solution.py
index 63d7aa7..5eca05f 100644
--- a/rounds/1_histogram/solution.py
+++ b/rounds/1_histogram/solution.py
@@ -5,24 +5,22 @@
 own faster implementation.
 """
 
+import numpy as np
+
 
 def compute_histogram(path: str) -> dict[bytes, int]:
     """Frequency of every 2-byte bigram in the file at ``path``."""
-    # Step 1: read the whole file into memory as a single bytes object.
     with open(path, "rb") as f:
         data = f.read()
 
-    # Create a 2D matrix to count bigrams
-    counts = [[0] * 256 for _ in range(256)]
+    arr = np.frombuffer(data, dtype=np.uint8)
+
+    # Vectorised bigram index: first_byte * 256 + second_byte
+    bigram_indices = arr[:-1].astype(np.uint16) * 256 + arr[1:]
 
-    for i in range(len(data) - 1):
-        # Increment the count in each cell
-        counts[data[i]][data[i + 1]] += 1
+    # Count every bigram in a single pass (C-level loop inside numpy)
+    counts = np.bincount(bigram_indices, minlength=65536)
 
-    # Convert the matrix to the original format
-    output = {}
-    for i in range(256):
-        for j in range(256):
-            if counts[i][j] > 0:
-                output[bytes([i, j])] = counts[i][j]
-    return output
+    # Build the result dict from non-zero entries only
+    nonzero = np.flatnonzero(counts)
+    return {int(idx).to_bytes(2, "big"): int(counts[idx]) for idx in nonzero}
diff --git a/rounds/3_dna/solution.py b/rounds/3_dna/solution.py
index 4507c2d..0756188 100644
--- a/rounds/3_dna/solution.py
+++ b/rounds/3_dna/solution.py
@@ -10,15 +10,18 @@
 import os
 from concurrent.futures import ThreadPoolExecutor
 
-_NL = 0x0A  # b"\n"
+_NL = 0x0A
 
 
 def find_matches(fasta_path: str, pattern: bytes) -> list[tuple[str, list[int]]]:
+    """Find every FASTA record whose sequence contains ``pattern``.
+
+    Returns ``[(record_id, [positions...]), ...]`` in file order.
+    """
     with open(fasta_path, "rb") as f:
         data = f.read()
 
-    # Step 1: locate every record start. A record starts with ``>`` either at
-    # offset 0 or immediately after a ``\n``.
+    # Step 1: locate every record start ('>' at offset 0 or after '\n').
     starts: list[int] = []
     i = 0
     while True:
@@ -28,14 +31,13 @@ def find_matches(fasta_path: str, pattern: bytes) -> list[tuple[str, list[int]]]
         if p == 0 or data[p - 1] == _NL:
             starts.append(p)
         i = p + 1
-    starts.append(len(data))  # sentinel marking the end of the last record.
+    starts.append(len(data))  # sentinel
 
     num_records = len(starts) - 1
     if num_records <= 0:
         return []
 
-    # Step 2: parallel scan. Choose enough batches to keep workers balanced
-    # even when record sizes vary.
+    # Step 2: parallel scan with batched work units.
     n_workers = max(1, os.cpu_count() or 1)
     batches = max(1, n_workers * 4)
     batch_size = max(1, (num_records + batches - 1) // batches)
@@ -46,15 +48,11 @@ def scan_batch(start_idx: int, end_idx: int) -> list[tuple[int, str, list[int]]]
             rec_start = starts[j]
             rec_end = starts[j + 1]
 
-            # Locate the end of the header line within this record's slice.
             nl = data.find(b"\n", rec_start, rec_end)
             if nl <= rec_start:
-                continue  # Malformed or header-only.
-
-            record_id = data[rec_start + 1 : nl].decode("ascii").strip()
+                continue
 
-            # Contiguous sequence: drop the newlines so matches that straddle
-            # line breaks are still found by ``bytes.find``.
+            # Strip newlines so cross-line matches are found.
             sequence = data[nl + 1 : rec_end].replace(b"\n", b"")
 
             positions: list[int] = []
@@ -67,6 +65,7 @@ def scan_batch(start_idx: int, end_idx: int) -> list[tuple[int, str, list[int]]]
                 s = p + 1
 
             if positions:
+                record_id = data[rec_start + 1 : nl].decode("ascii").strip()
                 out.append((j, record_id, positions))
         return out
 
@@ -77,8 +76,7 @@ def scan_batch(start_idx: int, end_idx: int) -> list[tuple[int, str, list[int]]]
         ]
         chunks = [f.result() for f in futures]
 
-    # Step 3: flatten and restore file order (record index is monotonic per
-    # batch, but batches finish in arbitrary order).
+    # Step 3: flatten and restore file order.
     flat = [item for chunk in chunks for item in chunk]
     flat.sort(key=lambda triple: triple[0])
     return [(rid, positions) for _, rid, positions in flat]