LeetCode: 114. Flatten Binary Tree to Linked List

LeetCode: 114. Flatten Binary Tree to Linked List

题目描述

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

题目大意: 将给定的二叉树拉直成链表形式。

解题思路 —— 递归求解

先将左子树和右子树拉直成链表形式，然后将它们合并成一条链表。如：

• 原始二叉树



• ​​flatten(root->left)​​: 拉直左子树



• ​​flatten(root->right)​​：拉直右子树



• 合并左右子树
  将右子树链接到左子树：



• 将左子树移动到右边：

AC 代码

/**
 * 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) {
        if(root == nullptr) return;

        // 分别对左右孩子处理
        flatten(root->left);
        flatten(root->right);

        // 将右边孩子链接在左孩子后面
        // 然后将左孩子移到右边
        TreeNode** leftChildTreeLeaf = &(root->left);
        while(*leftChildTreeLeaf != nullptr)
        {
            leftChildTreeLeaf = &(*leftChildTreeLeaf)->right;
        }
        *leftChildTreeLeaf = root->right;
        root->right = root->left;
        root->left = nullptr;
    }
};