[leetcode]Convert Sorted Array to Binary Search Tree

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<int> &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<int> &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);
    }
};

EOF