給定一個二叉樹径缅，原地將它展開為鏈表宣肚。
例如
給定二叉樹
1
/ \
2 5
/ \ \
3 4 6
將其展開為：
1
\
2
\
3
\
4
\
5
\
6
代碼
class Solution {
public:
void flatten(TreeNode *root) {
if (!root) return;
if (root->left) flatten(root->left);
if (root->right) flatten(root->right);
TreeNode *tmp = root->right;
root->right = root->left;
root->left = NULL;
while (root->right) root = root->right;
root->right = tmp;
}
};
