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• 原题连接：114. 二叉树展开为链表

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1- 思路

技巧——借助指针

思路：通过 ① 将左子树的右下结点的 .next ——> 拼接到当前节点的右子树上。

• 构造 cur 指针， cur = root
• 借助 pre 指针和 next 指针
  • pre 指针：用来定位左子树的最右下结点，用来拼接现有结点的右侧子树
  • next 指针：又来记录 cur.next 用来处理，用于 更新现有结点的右子树

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• 1- 第一个 while 条件 **while(cur != null)**
  • 1.1 如果左侧不为 null
    • 定义 next ，记录翻转后的 next 结点，next = cur.left
    • 定义 pre，利用 while，定位 pre 到最右下结点pre = next
• 2- 更新逻辑
  • 2.1 利用 pre 定位到 左子树的 最右下结点
  • 2.2 pre的 next 为 cur.next
  • 2.3 cur.right = next
  • 2.4 cur.left = null;
• 3- 最终移动 cur
  • cur = cur.next

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2- 实现

⭐114. 二叉树展开为链表——题解思路

class Solution {
    public void flatten(TreeNode root) {
        TreeNode cur = root;
        while(cur!=null){
            if(cur.left!=null){
                TreeNode next = cur.left;
                TreeNode pre = next;
                while(pre.right != null){
                    pre = pre.right;
                }
                // 更新逻辑
                pre.right = cur.right;
                cur.right = next;
                cur.left = null;
            }
            cur = cur.right;
        }

    }
}

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3- ACM 实现

public class falttern {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode() {}
        TreeNode(int val) { this.val = val; }
        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

    public static TreeNode build(String str){
        if(str == null || str.length()==0){
            return null;
        }
        String input = str.replace("[","");
        input = input.replace("]","");
        String[] parts = input.split(",");

        Integer[] nums = new Integer[parts.length];
        for(int i = 0 ; i < parts.length;i++){
            if(!parts[i].equals("null")){
                nums[i] = Integer.parseInt(parts[i]);
            }else{
                nums[i] = null;
            }
        }
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(nums[0]);
        queue.offer(root);
        int index = 1;
        while(!queue.isEmpty()&& index<parts.length){
            TreeNode node = queue.poll();
            if(index<nums.length && nums[index]!=null){
                node.left = new TreeNode(nums[index]);
                queue.offer(node.left);
            }
            index++;
            if(index<nums.length && nums[index]!=null){
                node.right = new TreeNode(nums[index]);
                queue.offer(node.right);
            }
            index++;
        }
        return root;
    }

    public static void flattern(TreeNode root){
        TreeNode cur = root;
        while(cur!=null){
            if(cur.left!=null){
                TreeNode pre = cur.left;
                TreeNode next = pre;
                while(pre.right!=null){
                    pre = pre.right;
                }
                // 处理逻辑
                pre.right = cur.right;
                cur.right = next;
                cur.left = null;
            }
            cur = cur.right;
        }
    }

    static List<List<Integer>> res = new ArrayList<>();
    public static List<List<Integer>> levelOrder(TreeNode root) {
        if(root==null){
            return res;
        }
// 队列
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int len = queue.size();
            List<Integer> path = new ArrayList<>();
            while(len>0){
                TreeNode nowNode = queue.poll();
                path.add(nowNode.val);
                if(nowNode.left!=null) queue.offer(nowNode.left);
                if(nowNode.right!=null) queue.offer(nowNode.right);
                len--;
            }
            res.add(new ArrayList<>(path));
        }
        return res;
    }


    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();
        TreeNode root = build(input);
        flattern(root);
        levelOrder(root);
        System.out.println("结果是"+res.toString());
    }
}