2641. Cousins in Binary Tree II

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class Solution {
    public TreeNode replaceValueInTree(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        root.val = 0;

        while (!queue.isEmpty()) {
            Queue<TreeNode> nextLevelQueue = new LinkedList<>();
            int nextLevelSum = 0;
            for (TreeNode node : queue) {
                if (node.left != null) {
                    nextLevelSum += node.left.val;
                    nextLevelQueue.offer(node.left);
                }
                if (node.right != null) {
                    nextLevelSum += node.right.val;
                    nextLevelQueue.offer(node.right);
                }
            }

            for (TreeNode node : queue) {
                int childSum = (node.left != null ? node.left.val : 0) + (node.right != null ? node.right.val : 0);
                if (node.left != null) {
                    node.left.val = nextLevelSum - childSum;
                }
                if (node.right != null) {
                    node.right.val = nextLevelSum - childSum;
                }
            }

            queue = nextLevelQueue;
        }

        return root;
    }
}

References

2641. Cousins in Binary Tree II