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:
-
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.
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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.
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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.
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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.
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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
/**
* 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);
}