https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list/

• 前序遍歷

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