114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

Hints:
If you notice carefully in the flattened tree, each node’s right child points to the next node of a pre-order traversal

/**
 * 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){
        if(!root) return;
        while(root){
            if(root->left && root->right){
                TreeNode* t = root->left;
                while(t->right) t = t->right;
                t->right = root->right;
                root->right = NULL;
            }
            if(root->left){
                root->right = root->left;
                root->left = NULL;
            }
            root= root->right;
        }
    }
};