【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);
}
}