[LeetCode] Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

思路：递归的思路。思路与重建二叉树类似。时间复杂度O(n), 空间复杂度O(logN)

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        if (num.empty()) return NULL;
        return sortedArrayToBST(num, 0, num.size() - 1);
    }
    TreeNode* sortedArrayToBST(vector<int> &num, int start, int last) {
        if (start > last) return NULL;
        
        int half = (start + last) >> 1;
        TreeNode* root = new TreeNode(num[half]);
        root->left = sortedArrayToBST(num,start, half - 1);
        root->right = sortedArrayToBST(num, half + 1, last);
        
        return root;
    }
};