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

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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.

5. 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.

6. 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:

/**
 * 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);
}