java: add «2641. Cousins in Binary Tree II»

URL:	https://leetcode.com/problems/cousins-in-binary-tree-ii/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/cousins-in-binary-tree-ii.java

@@ -0,0 +1,51 @@
+class Solution {
+  private int get(TreeNode node) {
+    if (node == null) {
+      return 0;
+    }
+    return node.val;
+  }
+
+  private void handleNode(Queue<TreeNode> q, int siblings, TreeNode node) {
+    if (node == null) {
+      return;
+    }
+
+    node.val = siblings;
+    q.offer(node);
+  }
+
+  private int handleLevel(Queue<TreeNode> q, int previous) {
+    int sum = 0;
+
+    for (int i = q.size(); i > 0; --i) {
+      var node = q.poll();
+      node.val = previous - node.val;
+
+      var siblings = get(node.left) + get(node.right);
+      handleNode(q, siblings, node.left);
+      handleNode(q, siblings, node.right);
+
+      sum += siblings;
+    }
+
+    return sum;
+  }
+
+  public TreeNode replaceValueInTree(TreeNode root) {
+    if (root == null) {
+      return root;
+    }
+
+    Queue<TreeNode> q = new ArrayDeque<>();
+
+    int sumAbove = root.val;
+    q.offer(root);
+
+    while (!q.isEmpty()) {
+      sumAbove = handleLevel(q, sumAbove);
+    }
+
+    return root;
+  }
+}