[LeetCode] 117. Populating Next Right Pointers in Each Node II 每个节点的右向指针 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

116. Populating Next Right Pointers in Each Node 的延续，如果给定的树是任何二叉树，不一定是完全二叉树，就是说不是每个节点都包含有两个子节点。

解法1：BFS, easy but not constant space, Complexity: time O(N) space O(N) - queue

解法2: Iteration - use dummy node to keep record of the next level's root to refer pre travel current level by referring to root in the level above，Complexity: time O(N) space O(1)

Java：BFS

class Solution {
    public void connect(TreeLinkNode root) {
        if(root == null)return;
        Queue<TreeLinkNode> nodes = new LinkedList<>();
        nodes.offer(root);
        while(!nodes.isEmpty()){
            int size = nodes.size();
            for(int i = 0; i < size; i++){
                TreeLinkNode cur = nodes.poll();
                TreeLinkNode n = null;
                if(i < size - 1){
                    n = nodes.peek();
                }
                cur.next = n;
                if(cur.left != null)nodes.offer(cur.left);
                if(cur.right != null)nodes.offer(cur.right);
            }
            
        }
    } 
}

Java: Iteration

class Solution {
    public void connect(TreeLinkNode root) {
        TreeLinkNode dummy = new TreeLinkNode(0);
        TreeLinkNode pre = dummy;//record next root
        while(root != null){
            if(root.left != null){
                pre.next = root.left;
                pre = pre.next;
            }
            if(root.right != null){
                pre.next = root.right;
                pre = pre.next;
            }
            root = root.next;//reach end, update new root & reset dummy
            if(root == null){
                root = dummy.next;
                pre = dummy;
                dummy.next = null;
            }
        }
    }
}　

Java:

public void connect(TreeLinkNode root) {
    if(root == null) 
        return;
 
    TreeLinkNode lastHead = root;//prevous level's head 
    TreeLinkNode lastCurrent = null;//previous level's pointer
    TreeLinkNode currentHead = null;//currnet level's head 
    TreeLinkNode current = null;//current level's pointer
 
    while(lastHead!=null){
        lastCurrent = lastHead;
 
        while(lastCurrent!=null){
            //left child is not null
            if(lastCurrent.left!=null)    {
                if(currentHead == null){
                    currentHead = lastCurrent.left;
                    current = lastCurrent.left;
                }else{
                    current.next = lastCurrent.left;
                    current = current.next;
                }
            }
 
            //right child is not null
            if(lastCurrent.right!=null){
                if(currentHead == null){
                    currentHead = lastCurrent.right;
                    current = lastCurrent.right;
                }else{
                    current.next = lastCurrent.right;
                    current = current.next;
                }
            }
 
            lastCurrent = lastCurrent.next;
        }
 
        //update last head
        lastHead = currentHead;
        currentHead = null;
    }
}

 Python:

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
        self.next = None
    
    def __repr__(self):
        if self is None:
            return "Nil"
        else:
            return "{} -> {}".format(self.val, repr(self.next))

class Solution:
    # @param root, a tree node
    # @return nothing
    def connect(self, root):
        head = root
        while head:
            prev, cur, next_head = None, head, None
            while cur:
                if next_head is None:
                    if cur.left:
                        next_head = cur.left
                    elif cur.right:
                        next_head = cur.right
                
                if cur.left:
                    if prev:
                        prev.next = cur.left
                    prev = cur.left
                    
                if cur.right:
                    if prev:
                        prev.next = cur.right
                    prev = cur.right
                    
                cur = cur.next
            head = next_head

C++:

class Solution {
public:
    void connect(TreeLinkNode *root) {
        TreeLinkNode *dummy = new TreeLinkNode(0), *t = dummy;
        while (root) {
            if (root->left) {
                t->next = root->left;
                t = t->next;
            }
            if (root->right) {
                t->next = root->right;
                t = t->next;
            }
            root = root->next;
            if (!root) {
                t = dummy;
                root = dummy->next;
                dummy->next = NULL;
            }
        }
    }
};

　　

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[LeetCode] 116. Populating Next Right Pointers in Each Node 每个节点的右向指针

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