114. Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:

    void preorder(TreeNode *root, vector<int> &pre) {
        if(root == NULL) return;
        pre.push_back(root->val);
        preorder(root->left, pre);
        preorder(root->right, pre);
    }

    void flatten(TreeNode* root) {
        if(root == NULL) return;

        vector<int> pre;
        preorder(root, pre);

        TreeNode *cur = root;

        for(int i = 0; i < pre.size(); i++) {
            cur->val = pre[i];
            cur->left = NULL;

            if(cur->right) cur = cur->right;
            else {
                if(i == pre.size() - 1) continue;
                TreeNode *new_node = new TreeNode();
                cur->right = new_node;
                cur = cur->right;
            }
        }
    }
};