LeetCode OJ

Array和List的区别在于前者可以随机访问，而后者只能顺序访问。对于把排好序的array转成BST，可以用top-down的方式，很直观也很自然,时间复杂度是O(n)。而对于List如果采用同样的方式，每次需要顺序遍历到中间节点，时间复杂度变成O(nlogn)，如果换一种思路，down-top，每次把左子树给先生成，然后是根节点，然后再是右子树，则不需要顺序遍历到中间节点，其时间复杂度就降到O(n)，下面是两种情况的AC代码：

/**
     * Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
     * list can be accessed in order only
     * @param head
     * @return
     */
    public TreeNode sortedListToBST(ListNode head){
        if(head == null)
            return null;
        int len = 0;
        ListNode temp = head;
        while(temp != null){
            len ++ ;
            temp = temp.next;
        }
        myHead = head;
        return downUpList2BST( 0 , len-1);           
    }
    ListNode myHead;
    private TreeNode downUpList2BST( int start, int end){
        if(start>end)
            return null;
        int mid = (start+end)/2;
        TreeNode left = downUpList2BST( start, mid-1);
        TreeNode root = new TreeNode(myHead.val);
        root.left = left;
        myHead = myHead.next;
        root.right = downUpList2BST(mid+1,end);
        return root;
    }
    /**
     * array can be accessed randomly
     * Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
     * @param num
     * @return
     */
    public TreeNode sortedArrayToBST(int[] num){
        if(num == null || num.length<=0)
            return null;
        return recSA2BST(0,num.length-1,num);
    }
    /**
     * 
     * @param start [
     * @param end ]
     * @param num
     * @return
     */
    private TreeNode recSA2BST(int start, int end, int[] num){
        if(start > end)
            return null;
        if(start == end)
        {
            TreeNode temp = new TreeNode(num[start]);
            return temp;
        }
        int mid = (start+end)/2;
        TreeNode root = new TreeNode(num[mid]);
        root.left = recSA2BST(start, mid-1, num);
        root.right = recSA2BST(mid+1,end,num);
        return root;
    }