update 0017.电话号码的字母组合.md about rust 优化

problems/0017.电话号码的字母组合.md

@@ -450,42 +450,31 @@ function letterCombinations(digits: string): string[] {
 ## Rust
 
 ```Rust
+const map: [&str; 10] = [
+    "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz",
+];
 impl Solution {
-    fn backtracking(result: &mut Vec<String>, s: &mut String, map: &[&str; 10], digits: &String, index: usize)  {
+    fn back_trace(result: &mut Vec<String>, s: &mut String, digits: &String, index: usize) {
         let len = digits.len();
         if len == index {
             result.push(s.to_string());
             return;
         }
-        // 在保证不会越界的情况下使用unwrap()将Some()中的值提取出来
-        let digit= digits.chars().nth(index).unwrap().to_digit(10).unwrap() as usize;
-        let letters = map[digit];
-        for i in letters.chars() {
+        let digit = (digits.as_bytes()[index] - b'0') as usize;
+        for i in map[digit].chars() {
             s.push(i);
-            Self::backtracking(result, s, &map, &digits, index+1);
+            Self::back_trace(result, s, digits, index + 1);
             s.pop();
         }
     }
     pub fn letter_combinations(digits: String) -> Vec<String> {
-        if digits.len() == 0 {
+        if digits.is_empty() {
             return vec![];
         }
-        const MAP: [&str; 10] = [
-        "",
-        "",
-        "abc",
-        "def",
-        "ghi",
-        "jkl",
-        "mno",
-        "pqrs",
-        "tuv",
-        "wxyz"
-        ];
-        let mut result: Vec<String> = Vec::new();
-        let mut s: String = String::new();
-        Self::backtracking(&mut result, &mut s, &MAP, &digits, 0);
-        result
+        let mut res = vec![];
+        let mut s = String::new();
+        Self::back_trace(&mut res, &mut s, &digits, 0);
+        res
     }
 }
 ```