[LeetCode] 031: Flatten Binary Tree to Linked List

[Problem]

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

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

[Solution]

/**
 * 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, TreeNode* &head, TreeNode* &tail){
        // null node
        if(root == NULL){
            head = NULL;
            tail = NULL;
    return;
        }
        
        // flatten the left brunch and the right brunch
        TreeNode *leftHead = NULL;
        TreeNode *leftTail = NULL;
        TreeNode *rightHead = NULL;
        TreeNode *rightTail = NULL;
        flatten(root->left, leftHead, leftTail);
        flatten(root->right, rightHead, rightTail);
        
        // set result
        if(leftHead == NULL){
            if(rightHead == NULL){
                root->left = NULL;
                root->right = NULL;
                head = root;
                tail = root;
            }
            else{
                root->left = NULL;
                root->right = rightHead;
                head = root;
                tail = rightTail;
            }
        }
        else{
            if(rightHead == NULL){
                root->left = NULL;
                root->right = leftHead;
                head = root;
                tail = leftTail;
            }
            else{
                root->left = NULL;
                root->right = leftHead;
                leftTail->right = rightHead;
                head = root;
                tail = rightTail;
            }
        }
    }
    // flatten a sub-tree
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       TreeNode *head = NULL;
       TreeNode *tail = NULL;
       flatten(root, head, tail);
    }
};

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