Convert sorted array into binary search tree

一开始用的第一种解法，但是大测试内存超了，所以不能迭代的时候不能每次返回一个TreeNode，而是需要建立一个引用，每一次修改引用可以节省空间。时间复杂度来说都是一样的。

class Solution {
public:

    TreeNode *build(vector<int> num, int start, int end){
        
        if (start > end) return NULL;
        
        int mid = (start + end) / 2;
        TreeNode *tmpRoot = new TreeNode(num[mid]);
        tmpRoot->left = build(num, start, mid-1);
        tmpRoot->right = build(num, mid+1, end);
        return tmpRoot;
    }
    
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         if (num.size() == 0) return NULL;
         
         int start = 0;
         int end = num.size()-1;
         return build(num,start,end);
    }
};



class Solution {  
public:  
    void buildTree(vector<int> &num, int start, int end, TreeNode* &tmpRoot)  
    {  
        if (start > end) return;  
          
        int mid = (start+end)/2;  
        tmpRoot = new TreeNode(num[mid]);  
        buildTree(num, start, mid-1, tmpRoot->left);  
        buildTree(num, mid+1, end, tmpRoot->right);  
          
    }  
      
    TreeNode *sortedArrayToBST(vector<int> &num) {  
        // Start typing your C/C++ solution below  
        // DO NOT write int main() function  
        TreeNode* root = NULL;  
        int start = 0;
        int end = num.size() - 1;
        buildTree(num, start, end, root);  
        return root;  
    }  
};