Given a binary tree

    struct TreeLinkNode {
      TreeLinkNode *left;
      TreeLinkNode *right;
      TreeLinkNode *next;
    }

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Note:

You may only use constant extra space.
You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).
For example,
Given the following perfect binary tree,

         1
       /  \
      2    3
     / \  / \
    4  5  6  7

After calling your function, the tree should look like:

         1 -> NULL
       /  \
      2 -> 3 -> NULL
     / \  / \
    4->5->6->7 -> NULL

这题需要在一棵完全二叉树中使用next指针连接旁边的节点，我们可以发现一些规律。

• 如果一个子节点是根节点的左子树，那么它的next就是该根节点的右子树，譬如上面例子中的4，它的next就是2的右子树5。
• 如果一个子节点是根节点的右子树，那么它的next就是该根节点next节点的左子树。譬如上面的5，它的next就是2的next（也就是3）的左子树。

所以代码如下：

class Solution {
public:
    void connect(TreeLinkNode *root) {
        if(!root) {
            return;
        }
        TreeLinkNode* p = root;
        TreeLinkNode* first = NULL;
        while(p) {
            //记录下层第一个左子树
            if(!first) {
                first = p->left;
            }
            //如果有左子树，那么next就是父节点
            if(p->left) {
                p->left->next = p->right;
            } else {
                //叶子节点了，遍历结束
                break;
            }
            //如果有next，那么设置右子树的next
            if(p->next) {
                p->right->next = p->next->left;
                p = p->next;
                continue;
            } else {
                //转到下一层
                p = first;
                first = NULL;
            }
        }
    }
};

Populating Next Right Pointers in Each Node II

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

You may only use constant extra space.
For example,
Given the following binary tree,

         1
       /  \
      2    3
     / \    \
    4   5    7

After calling your function, the tree should look like:

         1 -> NULL
       /  \
      2 -> 3 -> NULL
     / \    \
    4-> 5 -> 7 -> NULL

不同于上一题，这题的二叉树并不是完全二叉树，我们不光需要提供first指针用来表示一层的第一个元素，同时也需要使用另一个lst指针表示该层上一次遍历的元素。那么我们只需要处理好如何设置last的next指针就可以了。

代码如下:

class Solution {
public:
    void connect(TreeLinkNode *root) {
        if(!root) {
            return;
        }
        TreeLinkNode* p = root;
        TreeLinkNode* first = NULL;
        TreeLinkNode* last = NULL;
        while(p) {
            //设置下层第一个元素
            if(!first) {
                if(p->left) {
                    first = p->left;
                } else if(p->right) {
                    first = p->right;
                }
            }
            if(p->left) {
                //如果有last，则设置last的next
                if(last) {
                    last->next = p->left;
                }
                //last为left
                last = p->left;
            }
            if(p->right) {
                //如果有last，则设置last的next
                if(last) {
                    last->next = p->right;
                }
                //last为right
                last = p->right;
            }
            //如果有next，则转到next
            if(p->next) {
                p = p->next;
            } else {
                //转到下一层
                p = first;
                last = NULL;
                first = NULL;
            }
        }
    }
};

其实我们可以看到，第一题只是第二题的特例，所以第二题的解法也同样适用于第一题。