leetcode - Convert Sorted Array to Binary Search Tree

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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        if(nums.size() == 0) return NULL;
        int mid = nums[nums.size()/2];
        int i = 0;
        TreeNode* root = new TreeNode(mid);
        
        root->left = toBST(nums, 0, nums.size()/2 - 1);
        root->right = toBST(nums, nums.size()/2+1, nums.size()-1);
        
        return root;
    }
    TreeNode* toBST(vector<int>& nums, int begin, int end){
        if(begin > end) return NULL;
        int mid = nums[(begin+end)/2];
        TreeNode* root = new TreeNode(mid);
        root->left = toBST(nums, begin, (begin+end)/2 - 1);
        root->right = toBST(nums, (begin+end)/2+1, end);
        return root;
        
    }
};

将一个升序数组建立一个高度平衡的二叉查找树。 如果高度平衡，则根节点一定是中间的数。用递归。