LeetCode筆記：Convert Sorted List to Binary Search Tree

LeetCode筆記：Convert Sorted List to Binary Search Tree

一. 題目描述

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

二. 題目分析

這道題只要將列表轉化為一個數組，就可以使用和題目Convert Sorted Array to Binary Search Tree一樣的方法來進行，這種方法的時間復雜度為O(n)，空間復雜度為O(n^2)。

三. 示例代碼

#include 
#include 

using std::vector;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};


struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


class Solution
{
private:
    TreeNode* sortedArrayToBST(vector::iterator Begin,
                               vector::iterator End)
    {
        if (Begin == End)
                {
                return NULL;
                }

        vector::iterator HeadIte = Begin + (End - Begin) / 2;
        TreeNode *Head = new TreeNode(*HeadIte);

        TreeNode *LeftChild = sortedArrayToBST(Begin, HeadIte);
        TreeNode *RightChild = sortedArrayToBST(HeadIte + 1, End);

        Head->left = LeftChild;
        Head->right = RightChild;

        return Head;
    }

public:
    TreeNode* sortedListToBST(ListNode* head)
    {
        vector Nums;

        for (ListNode *TmpHead = head; TmpHead; TmpHead = TmpHead->next)
        {
            Nums.push_back(TmpHead->val);
        }

        return sortedArrayToBST(Nums.begin(), Nums.end());
    }
};

四. 小結

以上所使用方法比較簡單，但還有可提升的空間。