[chore] add string reversal

strings/reverse_string_words.ipynb

@@ -0,0 +1,85 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem\n",
+    "\n",
+    "Given a string of words delimited by spaces, reverse the words in string. For example, given \"hello world here\", return \"here world hello\"\n",
+    "\n",
+    "Follow-up: given a mutable string representation, can you perform this operation in-place?"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 28,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def reverse(string_list: str, start: int, end: int) -> str:\n",
+    "    while start < end:\n",
+    "        string_list[start], string_list[end] = string_list[end], string_list[start]\n",
+    "        start += 1\n",
+    "        end -= 1\n",
+    "\n",
+    "def reverse_words(string_list):\n",
+    "    reverse(string_list, 0, len(string_list) - 1)\n",
+    "    start = 0\n",
+    "    for i in range(len(string_list)):\n",
+    "        if string_list[i] == \" \":\n",
+    "            reverse(string_list, start, i - 1)\n",
+    "            start = i + 1\n",
+    "    # we need to reverse the last sub-string\n",
+    "    reverse(string_list, start, len(string_list) - 1)\n",
+    "    return \"\".join(string_list)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 29,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'there hi'"
+      ]
+     },
+     "execution_count": 29,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "reverse_words(list(\"hi there\"))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": []
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.7.6"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}