Update 0647.回文子串.md

problems/0647.回文子串.md

@@ -519,6 +519,24 @@ function expandRange(s: string, left: number, right: number): number {
 
 Rust：
 
+```rust
+impl Solution {
+    pub fn count_substrings(s: String) -> i32 {
+        let mut dp = vec![vec![false; s.len()]; s.len()];
+        let mut res = 0;
+
+        for i in (0..s.len()).rev() {
+            for j in i..s.len() {
+                if s[i..=i] == s[j..=j] && (j - i <= 1 || dp[i + 1][j - 1]) {
+                    dp[i][j] = true;
+                    res += 1;
+                }
+            }
+        }
+        res
+    }
+}
+```
 
 > 双指针