HashMaps

HashMap<K, V>

HashMap stores key-value pairs with O(1) average-case lookup.

Creating a HashMap

use std::collections::HashMap;

let mut scores: HashMap<String, i32> = HashMap::new();

Inserting and Accessing

scores.insert(String::from("Alice"), 10);
scores.insert(String::from("Bob"), 20);

// Access — panics if key missing
println!("{}", scores["Alice"]);

// Safe access
if let Some(score) = scores.get("Alice") {
    println!("{}", score);
}

Entry API

// Insert if not present
scores.entry(String::from("Alice")).or_insert(50);

// Modify existing or insert new
let count = map.entry(word).or_insert(0);
*count += 1;

Iterating

for (key, value) in &scores {
    println!("{}: {}", key, value);
}

Note: HashMap iteration order is non-deterministic. Do not rely on order.

Other Methods

scores.contains_key("Alice")   // true
scores.remove("Bob")           // removes, returns Option<V>
scores.len()                   // number of entries

Your Task

1. word_count(text: &str) -> HashMap<&str, usize> — count occurrences of each word.
2. two_sum(nums: &[i32], target: i32) -> Option<(usize, usize)> — find two indices that sum to target.
3. most_frequent(v: &[i32]) -> i32 — returns the most frequently occurring element.