java: add «1405. Longest Happy String»

URL:	https://leetcode.com/problems/longest-happy-string/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/longest-happy-string.java

@@ -0,0 +1,36 @@
+class Solution {
+  private char tryAppend(int[] allowed, int[] trailing, char letter) {
+    int i = letter - 'a';
+
+    if ((allowed[i] >= allowed[(i + 1) % 3]
+            && allowed[i] >= allowed[(i + 2) % 3]
+            && trailing[i] != 2)
+        || (allowed[i] > 0 && (trailing[(i + 1) % 3] == 2 || trailing[(i + 2) % 3] == 2))) {
+      --allowed[i];
+      ++trailing[i];
+      trailing[(i + 1) % 3] = 0;
+      trailing[(i + 2) % 3] = 0;
+      return letter;
+    }
+
+    return '\0';
+  }
+
+  public String longestDiverseString(int a, int b, int c) {
+    var longest = new StringBuilder();
+
+    var allowed = new int[] {a, b, c};
+    var trailing = new int[3];
+    for (int i = a + b + c; i > 0; --i) {
+      for (var letter : new char[] {'a', 'b', 'c'}) {
+        var toAppend = tryAppend(allowed, trailing, letter);
+        if (toAppend != '\0') {
+          longest.append(letter);
+          break;
+        }
+      }
+    }
+
+    return longest.toString();
+  }
+}