二叉树展开为链表

二叉树展开为链表

题目：
给定一个二叉树，原地将它展开为一个单链表。
例如，给定二叉树

将其展开为：

解题思路：运用前序遍历将节点添加进list中，然后遍历list改变每一个节点的左右节点

/**
 * 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 {
    private List<TreeNode> list = new ArrayList();
    public void flatten(TreeNode root) {
        if(root == null)
            return;
        
        preOrder(root);
        int len = list.size();
        for(int i = 1; i < len; i++) {
            TreeNode cur = list.get(i);
            TreeNode pre = list.get(i - 1);
            pre.left = null;
            pre.right = cur;
        }
    }
    
    private void preOrder(TreeNode node) {
        if(node == null)
            return ;
        
        list.add(node);
        preOrder(node.left);
        preOrder(node.right);
    }
}