add 2124

Author: fishercoder1534

README.md

@@ -8,6 +8,7 @@ _If you like this project, please leave me a star._ ★
 
 |  #  |      Title     |   Solutions   | Video  | Difficulty  | Tag                  
 |-----|----------------|---------------|--------|-------------|-------------
+|2124|[Check if All A's Appears Before All B's](https://leetcode.com/problems/check-if-all-as-appears-before-all-bs/)|[Java](../master/src/main/java/com/fishercoder/solutions/_2124.java) ||Easy||
 |2120|[Execution of All Suffix Instructions Staying in a Grid](https://leetcode.com/problems/execution-of-all-suffix-instructions-staying-in-a-grid/)|[Java](../master/src/main/java/com/fishercoder/solutions/_2120.java) ||Medium||
 |2119|[A Number After a Double Reversal](https://leetcode.com/problems/a-number-after-a-double-reversal/)|[Java](../master/src/main/java/com/fishercoder/solutions/_2119.java) ||Easy||
 |2116|[Check if a Parentheses String Can Be Valid](https://leetcode.com/problems/check-if-a-parentheses-string-can-be-valid/)|[Java](../master/src/main/java/com/fishercoder/solutions/_2116.java) ||Medium||

src/main/java/com/fishercoder/solutions/_2124.java

@@ -0,0 +1,18 @@
+package com.fishercoder.solutions;
+
+public class _2124 {
+    public static class Solution1 {
+        public boolean checkString(String s) {
+            int aIndex = s.lastIndexOf('a');
+            if (aIndex < 0) {
+                return true;
+            }
+            for (int i = 0; i < aIndex; i++) {
+                if (s.charAt(i) == 'b') {
+                    return false;
+                }
+            }
+            return true;
+        }
+    }
+}