[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.

解题思路：

递归做。

/**

 * 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 *createTree(vector<int> &num, int start, int end)

    {

        if(start > end)

            return NULL;

        int mid = (start + end) / 2;

        TreeNode *l = createTree(num, start, mid - 1);

        TreeNode *r = createTree(num, mid + 1, end);

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

        cur -> left = l;

        cur -> right = r;

        

        return cur;

    }

    

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

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

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

    }

};