assignment 1~5: fixes

- assignment05/pascal.mlw: lowered the difficulty (one more invariant given)
- assignment02, 03: minor fixes & divide into sub-problems

src/assignments/assignment03/small_problems.rs

@@ -0,0 +1,130 @@
+//! Small problems.
+
+use std::collections::{HashMap, HashSet};
+use std::fmt;
+
+/// Day of week.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DayOfWeek {
+    /// Sunday.
+    Sun,
+    /// Monday.
+    Mon,
+    /// Tuesday.
+    Tue,
+    /// Wednesday.
+    Wed,
+    /// Thursday.
+    Thu,
+    /// Friday.
+    Fri,
+    /// Saturday.
+    Sat,
+}
+
+/// The next day of week.
+///
+/// `next_weekday(Thu)` is `Fri`; and `next_weekday(Fri)` is `Mon`.
+pub fn next_weekday(day: DayOfWeek) -> DayOfWeek {
+    todo!()
+}
+
+/// Given a list of integers, returns its median (when sorted, the value in the middle position).
+///
+/// For a data set `x` of `n` elements, the median can be defined as follows:
+///
+/// - If `n` is odd, the median is `(n+1)/2`-th smallest element of `x`.
+/// - If `n` is even, the median is `(n/2)+1`-th smallest element of `x`.
+///
+/// For example, the following list of seven numbers,
+///
+/// ```ignore
+/// vec![1, 3, 3, 6, 7, 8, 9]
+/// ```
+///
+/// has the median of 6, which is the fourth value. And for this data set of eight numbers,
+///
+/// ```ignore
+/// vec![1, 2, 3, 4, 5, 6, 8, 9]
+/// ```
+///
+/// it has the median of 5, which is the fifth value.
+///
+/// Returns `None` if the list is empty.
+pub fn median(values: Vec<isize>) -> Option<isize> {
+    todo!()
+}
+
+/// Given a list of integers, returns its smallest mode (the value that occurs most often; a hash map will be helpful here).
+///
+/// Returns `None` if the list is empty.
+pub fn mode(values: Vec<isize>) -> Option<isize> {
+    todo!()
+}
+
+/// Converts the given string to Pig Latin. Use the rules below to translate normal English into Pig Latin.
+///
+/// 1. If a word starts with a consonant and a vowel, move the first letter of the word at the end of the word and add "ay".
+///
+/// Example: "happy" -> "appyh" + "ay" -> "appyhay"
+///
+/// 2. If a word starts with multiple consonants, move them to the end of the word and add "ay".
+///
+/// Example: "string" -> "ingstr" + "ay" -> "ingstray"
+///
+/// 3. If a word starts with a vowel, add the word "hay" at the end of the word.
+///
+/// Example: "explain" -> "explain" + "hay" -> "explainhay"
+///
+/// Keep in mind the details about UTF-8 encoding!
+///
+/// You may assume the string only contains lowercase alphabets, and it contains at least one vowel.
+pub fn piglatin(input: String) -> String {
+    todo!()
+}
+
+/// Converts HR commands to the organization table.
+///
+/// If the commands are as follows:
+///
+/// ```ignore
+/// vec!["Add Amir to Engineering", "Add Sally to Sales", "Remove Jeehoon from Sales", "Move Amir from Engineering to Sales"]
+/// ```
+///
+/// The return value should be:
+///
+/// ```ignore
+/// ["Sales" -> ["Amir", "Sally"]]
+/// ```
+///
+/// - The result is a map from department to the list of its employees.
+/// - An empty department should not appear in the result.
+/// - There are three commands: "Add {person} to {department}", "Remove {person} from {department}", and "Move {person} from {department} to {department}".
+/// - If a command is not executable, then it's ignored.
+/// - There is no space in the name of the person and department.
+///
+/// See the test function for more details.
+pub fn organize(commands: Vec<String>) -> HashMap<String, HashSet<String>> {
+    todo!()
+}
+
+/// Events in a text editor.
+#[derive(Debug)]
+pub enum TypeEvent {
+    /// A character is typed.
+    Type(char),
+    /// The last character is removed.
+    Backspace,
+    /// The whole string is copied to the clipboard.
+    Copy,
+    /// The string in the clipboard is appended.
+    Paste,
+}
+
+/// Starting from an empty string and an empty clipboard,
+/// processes the given `events` in order and returns the resulting string.
+///
+/// See the test function `test_editor` for examples.
+pub fn use_editor(events: Vec<TypeEvent>) -> String {
+    todo!()
+}