116. Populating Next Right Pointers in Each Node

Description

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,

tree

After calling your function, the tree should look like:

linked tree

Solution

Level-order-traversal, time O(n), space O(n)

层序遍历的小小变种。

/**
 * 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;
        }
        
        Queue queue = new LinkedList<>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            TreeLinkNode pre = null;
            
            while (levelSize-- > 0) {
                TreeLinkNode curr = queue.poll();
                if (pre != null) {
                    pre.next = curr;
                }
                
                pre = curr;
                
                if (curr.left != null) queue.offer(curr.left);
                if (curr.right != null) queue.offer(curr.right);
            }
        }
    }
}

Iterative, time O(n), space O(1)

扫描level时，连接level + 1。

/**
 * 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 curr = root;
        
        while (curr.left != null) {
            TreeLinkNode nextLevelStart = curr.left;
            
            while (curr != null) {
                curr.left.next = curr.right;
                if (curr.next != null) {
                    curr.right.next = curr.next.left;
                }
                
                curr = curr.next;
            }
            
            curr = nextLevelStart;
        }
    }
}