Leetcode:Flatten Binary Tree to Linked List 解题报告

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

SOLUTION 1:
使用递归解决，根据left是否为空，先连接left tree, 然后再连接右子树。使用一个tail 来记录链的结尾。在递归之前，先将root.left,root.right保存下来。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        dfs(root);
    }
    
    // return : the tail of the list.
    public TreeNode dfs(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        TreeNode left = root.left;
        TreeNode right = root.right;
        
        // Init the root.        
        root.left = null;
        root.right = null;
        
        TreeNode tail = root;
        
        // connect the left tree.
        if (left != null) {
            tail.right = left;
            tail = dfs(left);
        }
        
        // connect the right tree.
        if (right != null) {
            tail.right = right;
            tail = dfs(right);
        }
        
        return tail;
    }
}