/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void flatten(TreeNode *root) {        TreeNode*now = root;        while (now)        {            if(now->left)            {                // Find current node‘s prenode that links to current node‘s right subtree                TreeNode* pre = now->left;                while(pre->right)                {                    pre = pre->right;                }                pre->right = now->right;                // Use current node‘s left subtree to replace its right subtree(original right                 // subtree is already linked by current node‘s prenode                now->right = now->left;                now->left = NULL;            }            now = now->right;        }    }};