二叉樹展開為連結串列

題目：
給定一個二叉樹，原地將它展開為一個單鏈表。
例如，給定二叉樹

將其展開為：

解題思路：運用前序遍歷將節點新增進list中，然後遍歷list改變每一個節點的左右節點

/**
 * 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);
    }
}