109. Convert Sorted List to Binary Search Tree

1.問題

• 給予一個已經排序過的linked list, 轉換成高度平衡樹

2.想法

• 二分法

3.程式碼

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * 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:
    TreeNode* sortedListToBST(ListNode* head) {
        if (head == NULL) {
            return NULL;
        }
        vector<int> nodes;
        ListNode* curr = head;
        while (curr) {
            nodes.push_back(curr->val);
            curr = curr->next;
        }
        return buildTree(nodes, 0, nodes.size() - 1);
    }
private:
    TreeNode* buildTree(vector<int> &nodes, int left, int right){
        if(left > right) {
            return NULL;
        }
        int mid = (left + right) / 2;
        TreeNode* node = new TreeNode(nodes[mid]);
        node->left = buildTree(nodes, left, mid - 1);
        node->right = buildTree(nodes, mid + 1, right);
        return node;
    }
};

4.Performance