Feat/check if all as appears before all bs 2124

src/main/java/com/leetcode/easy/CheckIfAllAsAppearsBeforeAllBs2124.java

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+// Tags: String
+package com.leetcode.easy;
+
+public class CheckIfAllAsAppearsBeforeAllBs2124 {
+
+    /**
+     * Checks if all 'a' characters in the string appear before all 'b' characters.
+     * <p>
+     * Returns {@code true} if every 'a' in the string appears before every 'b'.
+     * If any 'a' appears after a 'b', returns {@code false}.
+     * <p>
+     * <b>Time Complexity: O(n)</b>
+     * <ul>
+     *   <li>n = length of the string</li>
+     *   <li>Single pass through the string: O(n)</li>
+     * </ul>
+     * <p>
+     * <b>Space Complexity: O(1)</b>
+     * <ul>
+     *   <li>Only uses a single boolean flag: O(1)</li>
+     * </ul>
+     *
+     * @param s the string to check, containing only characters 'a' and 'b'
+     * @return {@code true} if all 'a's appear before all 'b's, {@code false} otherwise
+     */
+    public boolean checkString(String s) {
+        boolean bSeen = false;
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (c == 'b') {
+                bSeen = true;
+            } else {
+                if (bSeen) {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+}

src/test/java/com/leetcode/easy/CheckIfAllAsAppearsBeforeAllBs2124Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class CheckIfAllAsAppearsBeforeAllBs2124Test {
+    private CheckIfAllAsAppearsBeforeAllBs2124 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new CheckIfAllAsAppearsBeforeAllBs2124();
+    }
+
+    @Test
+    void testCheckString_Example1_AllABeforeB() {
+        // Input: s = "aaabbb"
+        // 'a' at indices 0,1,2; 'b' at indices 3,4,5
+        assertTrue(solution.checkString("aaabbb"));
+    }
+
+    @Test
+    void testCheckString_Example2_AAfterB() {
+        // Input: s = "abab"
+        // 'a' at index 2 appears after 'b' at index 1
+        assertFalse(solution.checkString("abab"));
+    }
+
+    @Test
+    void testCheckString_Example3_NoA() {
+        // Input: s = "bbb"
+        // No 'a's, so condition is satisfied
+        assertTrue(solution.checkString("bbb"));
+    }
+}