【LeetCode】Flatten Binary Tree to Linked List

题目：Flatten Binary Tree to Linked List

<span style="font-size:18px;">/**LeetCode Flatten Binary Tree to Linked List * 题意:给定一个二叉树，将其转变为一个相当于单链表的结构，观察可知该结构即：每一个节点左儿子为空，右儿子指向自己先序遍历时的下一个节点 * 思路：有观察可得，应对其进行先序遍历，得到正确的序列连接起来 * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */ package javaTrain;public class Train18 {public void flatten(TreeNode root) {if(root== null || (root.left == null && root.right == null)) return;preOrder(root);return;    }private TreeNode preOrder(TreeNode root){if(root== null || (root.left == null && root.right == null)) return root;TreeNode left = root.left;TreeNode right = root.right;TreeNode last;root.left = null; if(left != null){root.right = left;last = preOrder(left);last.left = null;if(right != null){last.right = right;return preOrder(right);}else return last;}else{root.right = right;return preOrder(right);}}}</span>