114. Flatten Binary Tree to Linked List

http://www.cnblogs.com/grandyang/p/4293853.html

用递归，先把左子树、右子树处理好，然后再将当前根节点和左、右子树进行处理。

处理的方式是将左子树换到右子树，且左子树为空，之前的右子树在整个新生成的右子树的最右节点

class Solution {
public:
    void flatten(TreeNode* root) {
        if(root == NULL)
            return;
        flatten(root->left);
        flatten(root->right);
        TreeNode* tmp = root->right;
        root->right = root->left;
        root->left = NULL;
        while(root->right)
            root = root->right;
        root->right = tmp;
    }
};