resolve aylei#17

Author: aylei

src/lib.rs

@@ -14,3 +14,4 @@ mod n0013_roman_to_integer;
 mod n0014_longest_common_prefix;
 mod n0015_3sum;
 mod n0016_3sum_closest;
+mod n0017_letter_combinations_of_a_phone_number;

src/n0017_letter_combinations_of_a_phone_number.rs

@@ -0,0 +1,67 @@
+/**
+ * [17] Letter Combinations of a Phone Number
+ *
+ * Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.
+ * 
+ * A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+ * 
+ * <img src="http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png" />
+ * 
+ * Example:
+ * 
+ * 
+ * Input: "23"
+ * Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
+ * 
+ * 
+ * Note:
+ * 
+ * Although the above answer is in lexicographical order, your answer could be in any order you want.
+ * 
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+impl Solution {
+    pub fn letter_combinations(digits: String) -> Vec<String> {
+        // '0' and '1' as placeholder to avoid index shifting
+        let table: Vec<(char, Vec<char>)> = vec![
+            ('0', vec![]),
+            ('1', vec![]),                ('2', vec!['a','b','c']), ('3', vec!['d','e','f']),
+            ('4', vec!['g','h','i']),     ('5', vec!['j','k','l']), ('6', vec!['m','n','o']),
+            ('7', vec!['p','q','r','s']), ('8', vec!['t','u','v']), ('9', vec!['w','x','y','z'])
+        ];
+        if digits.len() < 1 { return vec![] }
+        let mut combs: Vec<String> = vec![String::with_capacity(digits.len())];
+        for ch in digits.chars().into_iter() {
+            let chs = &table[ch.to_digit(10).unwrap() as usize].1;
+            let mut added: Vec<String> = Vec::with_capacity((chs.len() - 1) * combs.len());
+            for comb in combs.iter_mut() {
+                for (i, &alphabetic) in chs.iter().enumerate() {
+                    if i == chs.len() - 1 {
+                        comb.push(alphabetic);
+                    } else {
+                        let mut new_comb = (*comb).clone();
+                        new_comb.push(alphabetic);
+                        added.push(new_comb);
+                    }
+                }
+            }
+            combs.append(&mut added);
+        }
+        combs
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_17() {
+        assert_eq!(Solution::letter_combinations("23".to_string()), ["cf", "af", "bf", "cd", "ce", "ad", "ae", "bd", "be"]);
+    }
+}