LeetCode //C - 114. Flatten Binary Tree to Linked List

114. Flatten Binary Tree to Linked List

Given the root of a binary tree, flatten the tree into a "linked list":

  • The "linked list" should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
  • The "linked list" should be in the same order as a pre-order traversal of the binary tree.

Example 1:

Input: root = [1,2,5,3,4,null,6]
Output: [1,null,2,null,3,null,4,null,5,null,6]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [0]
Output: [0]

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -100 <= Node.val <= 100

From: LeetCode

Link: 114. Flatten Binary Tree to Linked List


Solution:

Ideas:
  1. Modified Pre-Order Traversal: Traditional pre-order traversal visits a node in the order: root, left subtree, and then right subtree. The modification here is that we're doing it in a slightly different order: we first flatten the right subtree, then the left subtree, and finally process the current root.

  2. Global prev Variable: This variable keeps track of the last node that we've visited. When we visit a new node, we'll be linking this node to the prev node using the right pointer.

  3. Flattening Process:

  • When we visit a node:
    • We recursively flatten its right subtree.
    • We recursively flatten its left subtree.
    • We then update the current node's right pointer to point to the prev node. This effectively appends the previously processed list to the current node.
    • We set the current node's left pointer to NULL (because we want the linked list to use the right pointers).
    • Finally, we update the prev node to be the current node, as this node will be the previous node for the next node we process.
  1. Resetting the prev Variable: Before starting the flattening process for a tree (or a subtree), we reset the prev variable to NULL. This ensures that the last node in the flattened list will correctly point to NULL instead of some node from a previous test case or a previous run.

  2. Auxiliary Recursive Function: We've split the logic into two functions:

  • flatten_recursive handles the actual recursive flattening logic.
  • flatten is the main function that resets the prev variable and then calls the recursive function to perform the flattening.
Code:
c 复制代码
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* prev = NULL;

void flatten_recursive(struct TreeNode* root) {
    if (!root) return;

    flatten_recursive(root->right);
    flatten_recursive(root->left);

    root->right = prev;
    root->left = NULL;
    prev = root;
}

void flatten(struct TreeNode* root) {
    prev = NULL;  // Reset the prev variable
    flatten_recursive(root);
}
相关推荐
想跑步的小弱鸡5 小时前
Leetcode hot 100(day 3)
算法·leetcode·职场和发展
xyliiiiiL7 小时前
ZGC初步了解
java·jvm·算法
爱的叹息7 小时前
RedisTemplate 的 6 个可配置序列化器属性对比
算法·哈希算法
独好紫罗兰8 小时前
洛谷题单2-P5713 【深基3.例5】洛谷团队系统-python-流程图重构
开发语言·python·算法
每次的天空9 小时前
Android学习总结之算法篇四(字符串)
android·学习·算法
请来次降维打击!!!9 小时前
优选算法系列(5.位运算)
java·前端·c++·算法
qystca9 小时前
蓝桥云客 刷题统计
算法·模拟
别NULL9 小时前
机试题——统计最少媒体包发送源个数
c++·算法·媒体
weisian15110 小时前
Java常用工具算法-3--加密算法2--非对称加密算法(RSA常用,ECC,DSA)
java·开发语言·算法
程序员黄同学11 小时前
贪心算法,其优缺点是什么?
算法·贪心算法