LeetCode – Refresh – Flatten Binary Tree To Linked List

Logic is simple, but careful with the details.

/**
 * 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) {
        if (!root) return;
        flatten(root->left);
        flatten(root->right);
        TreeNode *runner = root->left;
        if (!root->left) return;
        else {
            while (runner->right) runner = runner->right;
        }
        runner->right = root->right;
        root->right = root->left;
        root->left = NULL;
    }
};