114. Flatten Binary Tree to Linked List（难）

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

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
	void flatten(TreeNode* root) {
		TreeNode* now = root;
		while (now){
			if (now->left){
				TreeNode* pre = now->left;
				while (pre->right){
					pre = pre->right;
				}
				//将当前节点的右孩子连接到当前节点左孩子的的最右孩子
				pre->right = now->right;

				//当前节点的左孩子变为右孩子，左孩子置为NULL
				now->right = now->left;
				now->left = NULL;
			}
			now = now->right;
		}
	}
};