[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;
}
};