【leetcode hot 100 114】二叉树展开为链表

解法一：执行一次先序遍历，把元素放入list中，然后放回root中

/**

• Definition for a binary tree node.
• public 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;
• }
• } */ class Solution { List list = new ArrayList<>(); public void flatten(TreeNode root) { // 执行一次先序遍历，把元素放入list中,然后放回root中 preorder(root); TreeNode pre = root; for(int i=1;i<list.size();i++){ pre.right = list.get(i); pre.left = null; pre = pre.right; } } public void preorder(TreeNode root){ if(root==null){ return; } list.add(root); preorder(root.left); preorder(root.right); } }