# Exercise: String Manipulator Tool

**Concepts practiced:** String vs &str, ownership/borrowing, Vec operations, pattern matching
**Objective:** Build a CLI tool that performs various string operations

## Requirements:
- Create a `StringTool` struct that holds a collection of strings
- Implement methods:
  - `new()` - creates empty tool
  - `add_string(&mut self, s: String)` - adds string to collection
  - `longest(&self) -> Option<&str>` - returns longest string (or None if empty)
  - `contains_word(&self, word: &str) -> Vec<&str>` - returns strings containing the word
  - `to_uppercase_all(&mut self)` - converts all strings to uppercase in-place

## Success criteria:
- Code compiles without warnings
- Test with: adding "hello", "world", "rust programming"
- `longest()` should return "rust programming"
- `contains_word("o")` should return vec!["hello", "world", "rust programming"]
- After `to_uppercase_all()`, all strings should be uppercase

## Stretch goal:
Add a method `word_count(&self) -> HashMap<String, usize>` that counts total occurrences of each word across all strings

## Getting Started:
1. `cd exercises && cargo new string-manipulator`
2. Focus on ownership: when do you need `String` vs `&str`?
3. Think about borrowing: methods that read vs modify

Expected time: 45-60 minutes