Leetcode:Flatten Binary Tree to Linked List 解题报告

Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

SOLUTION 1:
使用递归解决，根据left是否为空，先连接left tree, 然后再连接右子树。使用一个tail 来记录链的结尾。在递归之前，先将root.left,root.right保存下来。

 /**
  * Definition for binary tree
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public void flatten(TreeNode root) {
         dfs(root);
     }

     // return : the tail of the list.
     public TreeNode dfs(TreeNode root) {
         if (root == null) {
             return null;
         }

         TreeNode left = root.left;
         TreeNode right = root.right;

         // Init the root.
         root.left = null;
         root.right = null;

         TreeNode tail = root;

         // connect the left tree.
         if (left != null) {
             tail.right = left;
             tail = dfs(left);
         }

         // connect the right tree.
         if (right != null) {
             tail.right = right;
             tail = dfs(right);
         }

         return tail;
     }
 }