BST创建

思路：把数字排序，排序后数字列，中间点为父节点，左边部分为左子树，右边为右子树。

假设节点定义为：

// Definition for binary tree
struct TreeNode {
   int val;
   TreeNode *left;
   TreeNode *right;
   TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

递归的思路实现：

TreeNode *makeArray2BST(int *a,int len)
{
    TreeNode *t;
    if(len == 0)
        return NULL;
    else if(len == 1)
    {
        t = new TreeNode(a[0]);
    }else if(len == 2){
        t = new TreeNode(a[1]);
        t->left = new TreeNode(a[0]);
    }else{
        int n = len/2;
        t = new TreeNode(a[n]);
        t->left = makeArray2BST(a,n);
        t->right = makeArray2BST(a+n+1,len-n-1);
    }
    return t;
}