add initial data

0017-letter-combinations-of-a-phone-number/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "leetcode"
+version = "0.1.0"

0017-letter-combinations-of-a-phone-number/Cargo.toml

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

0017-letter-combinations-of-a-phone-number/src/main.rs

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+pub fn letter_combinations(digits: String) -> Vec<String> {
+    let mut current = Vec::new();
+    current.push("".to_string());
+
+    let mut next = Vec::new();
+
+    for digit in digits.chars() {
+        for entry in current.drain(..) {
+            for letter in to_letter(digit) {
+                let mut this_entry = entry.clone();
+                this_entry.push(letter);
+                next.push(this_entry);
+            }
+        }
+        std::mem::swap(&mut current, &mut next);
+        next.clear();
+    }
+
+    current
+}
+
+fn to_letter(digit: char) -> impl Iterator<Item = char> {
+    let letters: &'static [char] = match digit {
+        '2' => &['a', 'b', 'c'],
+        '3' => &['d', 'e', 'f'],
+        '4' => &['g', 'h', 'i'],
+        '5' => &['j', 'k', 'l'],
+        '6' => &['m', 'n', 'o'],
+        '7' => &['p', 'q', 'r', 's'],
+        '8' => &['t', 'u', 'v'],
+        '9' => &['w', 'x', 'y', 'z'],
+        _ => panic!(),
+    };
+    letters.iter().copied()
+}
+
+fn main() {
+    println!("Hello, world!");
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashSet;
+
+    use crate::letter_combinations;
+
+    #[test]
+    fn test_1() {
+        let letters = letter_combinations("23".to_string())
+            .into_iter()
+            .collect::<HashSet<_>>();
+        for example in ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"] {
+            assert!(letters.contains(example));
+        }
+    }
+
+    #[test]
+    fn test_2() {
+        let letters = letter_combinations("2".to_string())
+            .into_iter()
+            .collect::<HashSet<_>>();
+        for example in ["a", "b", "c"] {
+            assert!(letters.contains(example));
+        }
+    }
+}