LeetCode 114. Flatten Binary Tree to Linked List

原题链接在这里：https://leetcode.com/problems/flatten-binary-tree-to-linked-list/description/

题目：

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

题解：

类似Flatten a Multilevel Doubly Linked List.

root.left == null时, 直接跳到root.right进行下一步.

root.left!=null 时, 目标就是把roo.left 这个子树的右边一条插进root 和 root.right之间.

Time Complexity: O(n).

Space: O(1).

AC Java:

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