Populating Next Right Pointers in Each Node II

Follow up for problem "Populating Next Right Pointers in Each Node".

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
Java代码：
public class Solution {
    public void connect(TreeLinkNode root) {
        
        TreeLinkNode cur=root;
TreeLinkNode head=root;
TreeLinkNode pre=null;

while(head!=null){
cur=head;
head=null;
pre=null;
while(cur!=null){

if(cur.left!=null){
if(pre!=null)pre.next=cur.left;
else head=cur.left;
pre=cur.left;
}

if(cur.right!=null){
if(pre!=null)pre.next=cur.right;
else head=cur.right;
pre=cur.right;
}
cur=cur.next;

}
}

}

时间复杂度O(N)
空间复杂度O(1)
思路:
在上一次已经连接好的情况下，再进行下一层的连接。