compute qubit indices in linear time instead of quadratic (#71)

Author: kevinsung

docs/qaoa/landscape_analysis.ipynb

@@ -640,8 +640,9 @@
         "def compute_energies(row):\n",
         "    permutation = []\n",
         "    qubit_map = {}\n",
+        "    final_qubit_index = {q: i for i, q in enumerate(row['final_qubits'])}\n",
         "    for i, q in enumerate(row['qubits']):\n",
-        "        fi = row['final_qubits'].index(q)\n",
+        "        fi = final_qubit_index[q]\n",
         "        permutation.append(fi)\n",
         "        qubit_map[i] = q\n",
         "        \n",