leetCode182|二叉树展开为链表

例如，给定二叉树

    1
   / \
  2   5
 / \   \
3   4   6
将其展开为：

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

import java.util.LinkedList;
import java.util.List;

public class FlattenTest2 {
    public static void main(String[] args) {
        TreeNode t1 = new TreeNode(1);
        TreeNode t2 = new TreeNode(2);
        TreeNode t3 = new TreeNode(5);
        TreeNode t4 = new TreeNode(3);
        TreeNode t5 = new TreeNode(4);
        TreeNode t6 = new TreeNode(6);
        t1.left = t2;
        t1.right = t3;
        t2.left = t4;
        t2.right = t5;
        t3.right = t6;
        flatten(t1);
        System.out.println("t1 = " + t1);
    }

    public static  void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        LinkedList<TreeNode> list = new LinkedList<>();
        dfs(root, list);
        TreeNode head = list.removeFirst();
        head.left = null;
        while (list.size() > 0) {
            TreeNode tempNode = list.removeFirst();
            tempNode.left = null;
            head.right = tempNode;
            head = head.right;

        }


    }

    private static  void dfs(TreeNode root, List<TreeNode> list) {
        if (root == null) {
            return;
        }
        list.add(root);
        if (root.left != null) {
            dfs(root.left, list);
        }
        if (root.right != null) {
            dfs(root.right, list);
        }
    }
}