leetcode144.二叉树展开为链表

思路源自 

leetcode-树篇 114题 二叉树展开为链表

 对于当前结点来说，先把左子树全部拉平成链表，再把右子树也全部拉平成链表

然后记录当前结点的右子树起始点temp，把当前结点右指针指向左子树，左子树的最右下边的结点的右指针指向一开始记录的temp，最后就拉平成链表了

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
     public void flatten(TreeNode root) {
        if (root != null) {
            flatten(root.left);
            flatten(root.right);
            TreeNode temp = root.right;
            root.right=root.left;
            root.left = null;
            while (root.right!=null) root=root.right;
            root.right = temp;
        }
    }
}