Leetcode114. 二叉树展开为链表

题目传送：https://leetcode.cn/problems/flatten-binary-tree-to-linked-list/

运行效率

代码如下

class Solution {
   public void flatten(TreeNode root) {
        TreeNode head = new TreeNode();
        List<TreeNode> list = preOrder(root);
        for (int i = 0; i < list.size()-1; i++) {
            TreeNode cur = list.get(i);
            TreeNode next = list.get(i + 1);
            cur.left = null;
            cur.right=next;
        }
         if(!list.isEmpty()){
            list.get(list.size()-1).right=null;
        }
    }

//先序遍历结果
    public List<TreeNode> preOrder(TreeNode root) {
        List<TreeNode> result = new ArrayList<>();
        //处理边界情况
        if (root == null) {
            return result;
        }
        //如果是单节点
        if (root.left == null && root.right == null) {
            result.add(root);
            return result;
        }
        //1、先添加根节点
        result.add(root);
        //2、再添加左子树
        if (root.left != null) {
            List<TreeNode> list = preOrder(root.left);
            result.addAll(list);
        }
        //2、最后添加右子树
        if (root.right != null) {
            List<TreeNode> list = preOrder(root.right);
            result.addAll(list);
        }
        return result;
    }
}
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