Leetcode 114. Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * 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;
        }
        vector<TreeNode*> nodes;
        preorderTranverse(nodes, root);
        for(int i = 0; i < nodes.size() - 1; i++) {
            nodes[i]->left = nullptr;
            nodes[i]->right = nodes[i + 1];
        }
        nodes[nodes.size() - 1]->left = nullptr;
        nodes[nodes.size() - 1]->right = nullptr;
    }

private:
    void preorderTranverse(vector<TreeNode*>& nodes, TreeNode* root) {
        if(!root) {
            return;
        }
        nodes.push_back(root);
        preorderTranverse(nodes, root->left);
        preorderTranverse(nodes, root->right);
    }
};