117. Populating Next Right Pointers in Each Node II

,

BFS

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class Solution {
    public Node connect(Node root) {
        if (root == null) {
            return null;
        }
        Queue<Node> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            Node pre = null;
            for (int i = queue.size(); i > 0; i--) {
                Node node = queue.poll();
                if (pre != null) {
                    pre.next = node;
                }

                // 从左至右
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
                pre = node;
            }
        }

        return root;
    }
}

BFS

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class Solution {
    public Node connect(Node root) {
        if (root == null) {
            return null;
        }

        Queue<Node> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            Node pre = null;
            for (int i = queue.size(); i > 0; i--) {
                Node node = queue.poll();
                node.next = pre;
                pre = node;

                // 从右至左
                if (node.right != null) {
                    queue.add(node.right);
                }
                if (node.left != null) {
                    queue.add(node.left);
                }
            }
        }

        return root;
    }
}

Iterate

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class Solution {
    public Node connect(Node root) {
        Node leftmost = root;
        while (leftmost != null) {
            Node nextLayerDummyHead = new Node(); // 下一层的哑节点
            Node pre = nextLayerDummyHead, curr = leftmost;
            while (curr != null) { // 遍历当前层节点，并连接下一层节点
                if (curr.left != null) {
                    pre.next = curr.left;
                    pre = pre.next;
                }
                if (curr.right != null) {
                    pre.next = curr.right;
                    pre = pre.next;
                }

                curr = curr.next;
            }

            leftmost = nextLayerDummyHead.next; // 进入下一层
        }

        return root;
    }
}

References

117. Populating Next Right Pointers in Each Node II