leetcode 刷题之路 69 Flatten Binary Tree to Linked List

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.
二叉树转换，转换后的形式中节点顺序恰好是二叉树前序遍历后的顺序。
解法，对二叉树的左子树作转换操作，对二叉树的右子树作转换操作，然后将左子树转换结果拼接到根节点和右子树转换结果中间，执行过程如下图所示。

1
        / \
       2   5
      / \   \
     3   4   6

1
       / \
      2   5
       \    \
         3    6
           \
             4

1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

AC code：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
 {
 public:
	 void flatten(TreeNode *root)
	 {
        if(root==NULL)
            return;
        flatten(root->left);
        flatten(root->right);
        if(root->left!=NULL)
        {
            TreeNode *p=root->left;
            while(p->right!=NULL)
                p=p->right;
            p->right=root->right;
            root->right=root->left;
            root->left=NULL;
        }
	 }
 };