LeetCode:Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List
Total Accepted: 85094 
Total Submissions: 270845 
Difficulty: Medium
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

click to show hints.
Hints:
If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.
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java code:

/**
 * 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) {
        
        TreeNode cur = root;
        while(cur != null) {
            
            if(cur.left != null) {
                TreeNode pre = cur.left;
                while(pre.right != null) pre = pre.right; //      “  ”  
                
                pre.right = cur.right; //“  ”            
                cur.right = cur.left;
                cur.left = null;
            }
            cur = cur.right; //      
        }
    }
}