Yu's Coding Garden : leetcode Question 23: Convert Sorted Array to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
Analysis:
Because the requirement "height balanced", this problem becomes relative easy.
Just recursively use the middle element in the array as the root node of the current tree(sub tree).

    TreeNode* cbst(vector<int> &num, int st, int ed){

        if (st>ed){

            return NULL;

        }else{

            int mid = st+(ed-st)/2;

            TreeNode *bst=new TreeNode(num[mid]);

            bst->left = cbst(num,st,mid-1);

            bst->right = cbst(num,mid+1,ed);

            return bst;

        }

    }

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

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        if (num.size()==0){return NULL;}

        return cbst(num,0,num.size()-1);

    }

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