[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.

首先，对于一个序列，找到它的中间元素作为根节点。根节点的左子节点连接到用同样方法处理中间元素之前的序列得到的根节点，根节点的右子节点连接到用同样方法处理中间元素之后的序列得到的根节点。时间复杂度为O(n)。

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return makeTree(0,num.size()-1,&num);
    }
    TreeNode * makeTree(int l,int r,vector<int> *v)
    {
        if(l>r)
            return NULL;
        TreeNode * root;
        if(l == r)
        {
            root = new TreeNode(v->at(l));
            return root;
        }
        int mid = (l + r)/2;
        root = new TreeNode(v->at(mid));
        root->left = makeTree(l,mid-1,v);
        root->right = makeTree(mid+1,r,v);
        return root;
    }
};