Leetcode No.114 二叉树展开为链表（DFS）

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