直来直去的题目，注意用 l 和 r 两个下标来指示左右范围，并且用 if ( l > r ) 来作为递归出口，相比于用 start 和 len 来指示范围更优雅且不易出错。

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

 

 

 

 

 

 

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