添加 0017.电话号码的字母组合 0077.组合 0077.组合优化 0216.组合总和III Rust版本

添加 0017.电话号码的字母组合 0077.组合 0077.组合优化 0216.组合总和III Rust版本

problems/0017.电话号码的字母组合.md

@@ -454,6 +454,49 @@ function letterCombinations(digits: string): string[] {
 };
 ```
 
+## Rust
+
+```Rust
+impl Solution {
+    fn backtracking(result: &mut Vec<String>, s: &mut String, map: &[&str; 10], 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() {
+            s.push(i);
+            Self::backtracking(result, s, &map, &digits, index+1);
+            s.pop();
+        }
+    }
+    pub fn letter_combinations(digits: String) -> Vec<String> {
+        if digits.len() == 0 {
+            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
+    }
+}
+```
+
 ## C
 
 ```c