leetcode 第114题 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.

思路:递归实现。Linked List的结果是二叉树先序遍历的结果。

C++实现:

/**
 * 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;
        TreeNode *keepright = root->right;
        root->right = root->left;
        TreeNode *keepleft = root->left;
        TreeNode *pre = root;
        root->left = NULL;
        flatten(keepleft);
        while(pre->right != NULL) {
            pre = pre->right;
        }
        pre->right = keepright;
        flatten(keepright);
    }
};
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