题目：

题解：

class Solution {Node last = null, nextStart = null;public Node connect(Node root) {if (root == null) {return null;}Node start = root;while (start != null) {last = null;nextStart = null;for (Node p = start; p != null; p = p.next) {if (p.left != null) {handle(p.left);}if (p.right != null) {handle(p.right);}}start = nextStart;}return root;}public void handle(Node p) {if (last != null) {last.next = p;} if (nextStart == null) {nextStart = p;}last = p;}
}