思路：如果当前节点有左子树那么就将该节点左子树的右链接到该店的右边。

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode* root) {
        while(root){
            auto p=root->left;
            if(p){
                while(p->right) p=p->right;
                p->right=root->right;
                root->right=root->left;
                root->left=NULL;
            }
            root=root->right;
        }

    }
};