leetcode Convert Sorted Array to Binary Search Tree

利用更有序数组，构造平衡二叉树。

思路，递归，每次中间节点为根节点，然后递归获得左右子树。

/**

 * 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 *fun(vector<int> &num, int start, int end)

    {

        if (start > end) return NULL;

        int mid = (start + end) / 2;

        TreeNode *root = new TreeNode(num[mid]);

        

        root -> left = fun(num, start, mid - 1);

        root -> right = fun(num, mid + 1, end);

        return root;

    }

    TreeNode *sortedArrayToBST(vector<int> &num) {

        return fun(num, 0, num.size() - 1);

    }

};