[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);
    }
};