Update 0647.回文子串.md about rust

problems/0647.回文子串.md

@@ -517,8 +517,33 @@ function expandRange(s: string, left: number, right: number): number {
 }
 ```
 
+Rust：
 
 
+> 双指针
+
+```rust
+impl Solution {
+    pub fn count_substrings(s: String) -> i32 {
+        let mut res = 0;
+        for i in 0..s.len() {
+            res += Self::extend(&s, i, i, s.len());
+            res += Self::extend(&s, i, i + 1, s.len());
+        }
+        res
+    }
+
+    fn extend(s: &str, mut i: usize, mut j: usize, len: usize) -> i32 {
+        let mut res = 0;
+        while i < len && j < len && s[i..=i] == s[j..=j] {
+            res += 1;
+            i = i.wrapping_sub(1);
+            j += 1;
+        }
+        res
+    }
+}
+```
 
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