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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    TreeNode last = null;
    public void flatten(TreeNode root) {
        if(root == null){
            return ;
        }
        //递归过程
        flatten(root.left);
        flatten(root.right);
        //处理过程
        //将root的右子树放到root左子树的最右子树上，然后将root的左子树连到root.right
        //同时将左子树置空，然后将指针指向root.right,向下继续进行
        if(root.left != null){
            TreeNode pre = root.left;
            while(pre.right != null){
                pre = pre.right;
            }
            pre.right = root.right;
            root.right = root.left;
            root.left = null;
        }

        root = root.right;
    }
}