晕。竟然另一样的一道题。换成sorted array的话。找到中间位置更加方便了。

TreeNode *sortTree(vector<int> &num, int start, int len){

    if(len <= 0)    return NULL;

    int middle = len/2;

    TreeNode *root = new TreeNode(num[start+middle]);

    root->left = sortTree(num, start, middle);

    root->right = sortTree(num, start+middle+1, len-middle-1);

}

class Solution {

public:

    TreeNode *sortedArrayToBST(vector<int> &num) {

        return sortTree(num, 0, num.size());

    }

};