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LeetCode 114. Flatten Binary Tree to Linked List

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Question

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

For example, Given 

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

Explanation

分治法，左右子树递归以后，把左右根节点依次排到右边，左边为空即可。

Code

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