[LeetCode]Convert Sorted Array to Binary Search Tree

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

这道题要求根据一个有序的数组建立一棵二叉排序树。

利用折半的思想，构造根节点，再递归构造左右子树。递归出口是数组中只含一个元素。

TreeNode *sortedArrayToBST(vector<int> &num) {
        if (!num.size())
            return NULL;
        return buildBST(num, 0, num.size() - 1);
    }

    TreeNode* buildBST(vector<int> num, int l, int r){
        if (l <= r){
            int mid = (l + r) / 2;
            TreeNode* root = new TreeNode(num[mid]);
            root->left = buildBST(num, l, mid - 1);
            root->right = buildBST(num, mid + 1, r);
            return root;
        }
        else
            return NULL;
    }