Leetcode#114 Flatten Binary Tree to Linked List

先序遍历二叉树，递归展开。

别忘了将left设为NULL，如果忘掉的话可能报Runtime Error，而且这个RE很难查出原因。

代码：

 1 TreeNode *solve(TreeNode *root) {
 2   if (!root)  return NULL;
 3         
 4   TreeNode *leftHead = root->left;
 5   TreeNode *leftTail = solve(leftHead);
 6   TreeNode *rightHead = root->right;
 7   TreeNode *rightTail = solve(rightHead);
 8   TreeNode *tail = root;
 9         
10   // 这一句别忘了
11   tail->left = tail->right = NULL;
12         
13   if (leftHead) {
14     tail->right = leftHead;
15     tail = leftTail;
16   }
17   if (rightHead) {
18     tail->right = rightHead;
19     tail = rightTail;
20   }
21         
22   return tail;
23 }
24 
25 void flatten(TreeNode *root) {
26   solve(root);
27 }