convert-sorted-list-to-binary-search-tree_2(AC).cpp

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: a tree node
     */
    TreeNode *sortedListToBST(ListNode *head) {
        if (head == NULL) {
            return NULL;
        }
        
        ListNode *p = head;
        vector<int> A;
        while (p != NULL) {
            A.push_back(p->val);
            p = p->next;
        }
        return sortedArrayToBST(A);
    }
    
    TreeNode *sortedArrayToBST(vector<int> &A) {
        if (A.empty()) {
            return NULL;
        }
        return convert(A, 0, A.size() - 1);
    }
private:
    TreeNode* convert(vector<int> &a, int ll, int rr) {
        int mm = (ll + rr) / 2;
        TreeNode* root = new TreeNode(a[mm]);
        if (ll < mm) {
            root->left = convert(a, ll, mm - 1);
        }
        if (rr > mm) {
            root->right = convert(a, mm + 1, rr);
        }
        return root;
    }
};