Update 几道常见的子符串算法题.md

Author: Snailclimb

docs/dataStructures-algorithms/几道常见的子符串算法题.md

@@ -459,7 +459,7 @@ public class Main {
         return 0;
       }
     }
-    return flag == 1 ? res : -res;
+   return flag != 2 ? res : -res;
 
   }