114. Flatten Binary Tree to Linked List - Medium

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   /   2   5
 / \   3   4   6

The flattened tree should look like:

1
   2
       3
           4
               5
                   6

每个节点的右节点都是preorder traverse的下一个节点 -> 应该先 反preorder遍历 (即node.right->node.left->node) 到最后，从最后开始relink

time: O(n), space: O(height)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    TreeNode prev = null;

    public void flatten(TreeNode root) {
        if(root == null) {
            return;
        }

        flatten(root.right);
        flatten(root.left);

        root.right = prev;
        root.left = null;
        prev = root;
    }
}

原文地址：https://www.cnblogs.com/fatttcat/p/10202108.html