#19: add solution for exercise #17 in rust

rust/backtracking/0017-letter-combinations-of-a-phone-number.rs

@@ -0,0 +1,39 @@
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn letter_combinations(digits: String) -> Vec<String> {
+                if digits.is_empty() {return vec![]};
+
+        let phone_mapping: HashMap<char, &str> = [
+            ('2', "abc"),
+            ('3', "def"),
+            ('4', "ghi"),
+            ('5', "jkl"),
+            ('6', "mno"),
+            ('7', "pqrs"),
+            ('8', "tuv"),
+            ('9', "wxyz")
+        ].into();
+
+        let mut results: Vec<String> = vec![];
+        for digit in digits.chars() {
+            let letters = phone_mapping[&digit];
+            let mut cur_results = vec![];
+
+            for letter in letters.chars() {
+                if results.len() == 0 {
+                    cur_results.push(letter.to_string());
+                    continue;
+                }
+
+                for res in &results {
+                    cur_results.push(res.to_owned() + &letter.to_string());
+                }
+            }
+
+            results = cur_results;
+        }
+
+        results
+    }
+}