Populating Next Right Pointers in Each Node II

最新推荐文章于 2021-01-29 16:34:55 发布

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本文介绍了一种在二叉树中填充每个节点的next指针的方法，使得同一层级的相邻节点通过next指针连接起来。该方法仅使用常数额外空间，并适用于任意二叉树结构。

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,

After calling your function, the tree should look like:

         1 -> NULL
       /  \
      2 -> 3 -> NULL
     / \    \
    4-> 5 -> 7 -> NULL

思路：利用一个 ArrayList 来存储next level node. So the node.next is the node in the arraylist next node.
易错点：每一层一定要把 ArrayList 清空。

/**
 * 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;
        List<TreeLinkNode> cur = new ArrayList<TreeLinkNode>();
        cur.add(root);
        while(true){
            List<TreeLinkNode> pre = new ArrayList<TreeLinkNode>(cur);
            cur.clear();
            for(int i = 0; i < pre.size(); i++){
                if(i < pre.size() - 1){
                    pre.get(i).next = pre.get(i + 1);
                }else{
                    pre.get(i).next = null;
                }
            }
            for(TreeLinkNode node : pre){
                if(node.left != null)
                    cur.add(node.left);
                if(node.right != null)
                    cur.add(node.right);
            }
            if(cur.size() < 1)
                break;
        }
    }
}