【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<TreeNode> 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);
    }
}