[leetcode] Flatten Binary Tree to Linked List

题目（Tree DFS）

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.

 

题解：

比较有意思的一条tree的题

public class Solution {
    public void flatten(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p=root;
        
        while(p!=null||!stack.empty())
        {
            if(p.right!=null)
               stack.push(p.right);
               
            if(p.left!=null)
            {
                p.right=p.left;
                p.left=null;
            }
            else if (!stack.empty())
            {
                TreeNode temp = stack.pop();
                p.right=temp;
            }
            p=p.right;
        }
    }
}