Added Lexicographically Smallest String After Substring Operation.java

Author: varunu28

Medium/Lexicographically Smallest String After Substring Operation.java

@@ -0,0 +1,20 @@
+class Solution {
+    public String smallestString(String s) {
+        StringBuilder sb = new StringBuilder();
+        int idx = 0;
+        while (idx < s.length() && s.charAt(idx) == 'a') {
+            sb.append(s.charAt(idx++));
+        }
+        if (idx == s.length()) {
+            return s.substring(0, s.length() - 1) + 'z';
+        }
+        while (idx < s.length() && s.charAt(idx) != 'a') {
+            sb.append(getPreviousChar(s.charAt(idx++)));
+        }
+        return sb.append(s.substring(idx)).toString();
+    }
+    
+    private char getPreviousChar(char c) {
+        return (char) (c - 1);
+    }
+}