【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>