Populating Next Right Pointers in Each Node II ---LeetCode

/**
* Definition for binary tree with next pointer.
* public class TreeLinkNode {
*     int val;
*     TreeLinkNode left, right, next;
*     TreeLinkNode(int x) { val = x; }
* }
*/
public class Solution {
public void connect(TreeLinkNode root) {
if (root == null) return ;

TreeLinkNode lastHead = root;
TreeLinkNode lastCurr = null;
TreeLinkNode currHead = null;
TreeLinkNode current  = null;

while (lastHead != null) {
lastCurr = lastHead;

while (lastCurr != null) {
// left child is not null
if (lastCurr.left != null) {
if (currHead == null) {
currHead = lastCurr.left;
current  = lastCurr.left;
} else {
current.next = lastCurr.left;
current = current.next;
}
}
// right child is not null
if (lastCurr.right != null) {
if (currHead == null) {
currHead = lastCurr.right;
current  = lastCurr.right;
} else {
current.next = lastCurr.right;
current = current.next;
}
}
lastCurr = lastCurr.next;
}
lastHead = currHead;
currHead = null;
}
}
}