java: add «988. Smallest String Starting From Leaf»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/smallest-string-starting-from-leaf.java

@@ -0,0 +1,33 @@
+class Solution {
+  private String smallestFromLeaf(TreeNode node, StringBuilder sb) {
+    if (node == null) {
+      sb.reverse();
+      String reversed = sb.toString();
+      sb.reverse();
+
+      return reversed;
+    }
+
+    sb.append((char) ('a' + node.val));
+    String left = smallestFromLeaf(node.left, sb);
+    String right = smallestFromLeaf(node.right, sb);
+    sb.deleteCharAt(sb.length() - 1);
+
+    if (node.left == null) {
+      return right;
+    } else if (node.right == null) {
+      return left;
+    }
+
+    int cmp = left.compareTo(right);
+    if (cmp < 0) {
+      return left;
+    }
+
+    return right;
+  }
+
+  public String smallestFromLeaf(TreeNode root) {
+    return smallestFromLeaf(root, new StringBuilder(8500));
+  }
+}