feat(recursion-solutions): add count substrings with matching start and end

Author: PavithraSharma14

14-recursion/solutions/CountSubstrings.java

@@ -0,0 +1,36 @@
+/*
+PROBLEM:
+Count substrings that start and end with same character.
+
+APPROACH:
+Use recursion with shrinking window.
+Compare first and last characters.
+*/
+
+public class CountSubstrings {
+
+    public static int count(String str, int i, int j) {
+        if (i > j) {
+            return 0;
+        }
+
+        if (i == j) {
+            return 1;
+        }
+
+        int res = count(str, i + 1, j)
+                + count(str, i, j - 1)
+                - count(str, i + 1, j - 1);
+
+        if (str.charAt(i) == str.charAt(j)) {
+            res++;
+        }
+
+        return res;
+    }
+
+    public static void main(String[] args) {
+        String str = "aba";
+        System.out.println(count(str, 0, str.length() - 1));
+    }
+}