108.Convert Sorted Array to Binary Search Tree(Array; Divide-and-Conquer, dfs)

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

思路：使用二分法，将list的中间节点作为根节点，然后分别处理list左半边及右半边，以此递归。

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;
        TreeNode* root = new TreeNode();
        dfs(num,,num.size()-,root);
        return root;
    }
    void dfs(vector<int> &num,int start, int end,TreeNode* treeNode)
    {
        int size = end-start+;
        if(size == )
        {
            treeNode->val = num[start];
            return;
        }
        int mid = size/ + start;
        treeNode->val = num[mid];
        treeNode->left = new TreeNode();
        dfs(num,start,mid-,treeNode->left);
        if(mid+<=end)
        {
            treeNode->right = new TreeNode();
            dfs(num,mid+,end,treeNode->right);
        }
    }
};