java: add «2415. Reverse Odd Levels of Binary Tree»

URL:	https://leetcode.com/problems/reverse-odd-levels-of-binary-tree/

java/reverse-odd-levels-of-binary-tree.java

@@ -0,0 +1,39 @@
+class Solution {
+  private void traverse(ArrayList<ArrayList<TreeNode>> nodes, int level, TreeNode node) {
+    if (node == null) {
+      return;
+    }
+
+    if (level % 2 == 1) {
+      var idx = level >> 1;
+      while (nodes.size() <= idx) {
+        nodes.addLast(new ArrayList<TreeNode>());
+      }
+
+      nodes.get(idx).addLast(node);
+    }
+
+    traverse(nodes, level + 1, node.left);
+    traverse(nodes, level + 1, node.right);
+  }
+
+  private void reverse(ArrayList<TreeNode> level) {
+    int i, j;
+    for (i = 0, j = level.size() - 1; i < j; ++i, --j) {
+      var tmp = level.get(i).val;
+      level.get(i).val = level.get(j).val;
+      level.get(j).val = tmp;
+    }
+  }
+
+  public TreeNode reverseOddLevels(TreeNode root) {
+    var nodes = new ArrayList<ArrayList<TreeNode>>();
+    traverse(nodes, 0, root);
+
+    for (var level : nodes) {
+      reverse(level);
+    }
+
+    return root;
+  }
+}