114. 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

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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 {
    //post order
    TreeNode pre = null;
    public void flatten(TreeNode root) {
        if(root == null)
            return;
        flatten(root.right);
        flatten(root.left);
        root.right = pre;
        root.left = null;
        pre = root;
    }
}