Given a binary tree, flatten it to a linked list in-place. http://leetcode.com/onlinejudge

leetcode_flatten_binary_tree_to_linked_list.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* flatten_(TreeNode* root) {
        if (root->left == NULL && root->right == NULL)
            return root;
        if (root->left == NULL)
            return flatten_(root->right);
        if (root->right == NULL) {
            TreeNode* tmp = flatten_(root->left);
            root->right = root->left;
            root->left = NULL;
            return tmp;
        }
        TreeNode* tmp = flatten_(root->left);
        tmp->right = root->right;
        root->right = root->left;
        root->left = NULL;
        return flatten_(root->right);
    }

    void flatten(TreeNode *root) {
        if (root == NULL)
            return;
        flatten_(root);
    }
};