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

• 很简单，先序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
void flatten(struct TreeNode* root) {
    struct TreeNode *tmp = NULL, *next = NULL;
    if(root == NULL)
        return NULL;
    if(root->left != NULL)
    {
        tmp = root->right;
        root->right = root->left;
        flatten(root->left);
        root->left = NULL;
        while(root->right != NULL)
            root = root->right;
        root->right = tmp;
        flatten(tmp);
    }
    else
        flatten(root->right);
}