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/**
 * Definition for a binary tree node.
 * 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 *curr = root, *prev = NULL;
        while (curr != NULL) {
            if (curr->left != NULL) {
                prev = curr->left;
                while(prev->right != NULL) prev = prev->right;
                prev->right = curr->right;
                curr->right = curr->left;
                curr->left = NULL;
            }
            curr = curr->right;
        }
    }
};