LintCode 把排序数组转换为高度最小的二叉搜索树

题目描述：
给一个排序数组（从小到大），将其转换为一棵高度最小的排序二叉树。

注意事项

There may exist multiple valid solutions, return any of them.

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样例
给出数组 [1,2,3,4,5,6,7], 返回

4
/ \
2 6
/ \ / \
1 3 5 7

ac代码：

/**
 * 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 A: A sorted (increasing order) array
     * @return: A tree node
     */
    TreeNode *dfs(vector<int> A,int l,int r)
    {
        if(l>r)
            return NULL;
        int mid=(r+l)/2;
        TreeNode *root=new TreeNode(A[mid]);
        root->left=dfs(A,l,mid-1);
        root->right=dfs(A,mid+1,r);
        return root;
    }
    TreeNode *sortedArrayToBST(vector<int> &A) {
        // write your code here
        if(A.size()==0)
            return NULL;
        int r,l,mid;
        l=0,r=A.size()-1;
        mid=(r+l)/2;
        TreeNode *root=new TreeNode(A[mid]);
        root->left=dfs(A,l,mid-1);
        root->right=dfs(A,mid+1,r);
        return root;
    }
};