Convert Sorted Array to Binary Search Tree

思想：二分法建立平衡二叉树，和用链表建立的思想一样，处理过程大致相同

/**
 * 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 *BalanceBst(vector<int> &num, int beg, int end){
              if(beg > end)
              return NULL;
              int mid = (beg+end)>>1;
              TreeNode *root = new TreeNode(num[mid]);
              root->left = BalanceBst(num, beg, mid-1);
              root->right = BalanceBst(num, mid+1, end);
              return root;
    }
   
    TreeNode *sortedArrayToBST(vector<int> &num) {
         // IMPORTANT: Please reset any member data you declared, as
         // the same Solution instance will be reused for each test case.
              return BalanceBst(num, 0, num.size()-1);
    }
};