问题

分析

class Solution {
    public Node connect(Node root) {
        if (root == null) {
            return null;
        }
        Node res = root;
		// 外层循环为树高的迭代
        while (root != null) {
            Node nextLayerHead = new Node(0);
            Node nextLayerNode = nextLayerHead;
            // 内层循环根据当前层已有的链表构建下一层的链表
            while (root != null) {
                if (root.left != null) {
                    nextLayerNode.next = root.left;
                    nextLayerNode = nextLayerNode.next;
                }
                if (root.right != null) {
                    nextLayerNode.next = root.right;
                    nextLayerNode = nextLayerNode.next;
                }
                root = root.next;
            }
            root = nextLayerHead.next;
        }
        return res;
    }
}