LeetCode --- Convert Sorted Array to Binary Search Tree

题目链接

Problem discription:

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

Accepted Code:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num, int start, int end) {
        if (start > end) {
            return NULL;
        } 
        // if you write " int middle = start + (end - start) >> 1"
        // instead of the following sentence, you should keep in 
        // mind that the priority of ">>" is weaker than "+" operator
        int middle = start + (end - start) / 2;
        TreeNode *p = new TreeNode(num[middle]);
        // elements at the left of num[middle] construct
        // the left child tree of "p" and its right child
        // tree consists of elements located at the right
        // of num[middle]...
        // because the given array is sorted
        if (start != end) {
            p->left = sortedArrayToBST(num, start, middle - 1);
            p->right = sortedArrayToBST(num, middle + 1, end);
        }
        return p;
    }
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // call the overload function
        return sortedArrayToBST(num, 0, num.size() - 1);
    }
};