Leetcode(114) Flatten Binary Tree to Linked List

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Description

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

For example, given the following tree:

1
2
3
4
5
    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
2
3
4
5
6
7
8
9
10
11
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

解法

刚开始没弄明白原地展开是什么鬼，意思是把树的左边往右边合并，思路是递归地解决问题，采用头递归地写法更好理解，首先对跟节点的左右子树分别递归地进行展开，得到的情况如下图所示：

1
2
3
4
5
6
7
 1
 / \
2   5
 \   \
  3   6
   \    
    4

之后在把根的左子树设为空，并找到原左子树的最下节点与右子树接上即可。

具体代码如下：

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        if(root == null){
            return;
        }
        if(root.left != null){
            flatten(root.left);
        }
        if(root.right != null){
            flatten(root.right);
        }
        TreeNode tmp = root.right;
        root.right = root.left;
        root.left = null;
        while(root.right != null){
            root = root.right;
        }
        root.right = tmp;
    }
    
}