116. Populating Next Right Pointers in Each Node(unsolved)

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

/** * Definition for binary tree with next pointer. * struct TreeLinkNode { *  int val; *  TreeLinkNode *left, *right, *next; *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {} * }; */class Solution {public:    void connect(TreeLinkNode *root) {        if(root==NULL) return;        auto pre=root;        while(pre->left)        {            auto cur=pre;            while(cur)            {                cur->left->next=cur->right;                if(cur->next) cur->right->next=cur->next->left;                cur=cur->next;            }            pre=pre->left;        }    }};

二刷时是这么做的：
用到了递归，然后都是从最左边往右传递

/** * Definition for binary tree with next pointer. * struct TreeLinkNode { *  int val; *  TreeLinkNode *left, *right, *next; *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {} * }; */class Solution {public:    void connect(TreeLinkNode *root) {        if(!root) return ;        return solve(root);    }    void solve(TreeLinkNode *root){        if(!root) return;        TreeLinkNode *cur=root ;        while(cur){        if(cur->left) {cur->left->next=cur->right;}        if(cur->right&&cur->next) {cur->right->next=cur->next->left;}        cur=cur->next;        }        solve(root->left);    }};};