feat: add new coding challenge to have fun

Author: 1995parham

multimap/.gitignore

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+/target

multimap/Cargo.toml

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+[package]
+name = "multimap"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

multimap/phonebook.txt

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+1 Hugo
+1 Hugo
+2 Hello
+2 Bye
+10 Find

multimap/src/main.rs

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+use std::{collections::HashMap, error::Error, fs};
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let content = fs::read_to_string("phonebook.txt")?;
+    let mut store: HashMap<i32, Vec<&str>> = HashMap::new();
+
+    for line in content.lines() {
+        let (number, name) = line
+            .split_once(' ')
+            .ok_or_else(|| format!("malformed line: {line:?}"))?;
+        let key: i32 = number.parse()?;
+        store.entry(key).or_default().push(name);
+
+        println!("{name} -> {number}");
+    }
+
+    println!("{:?}", store.get(&99));
+    println!("{:?}", store.get(&1));
+    println!("{:?}", store.get(&2));
+
+    Ok(())
+}