LeetCode :: Convert Sorted Array (link list) to Binary Search Tree [tree]

標籤：leetcode   c++   遞迴   tree   

1.Given an array where elements are sorted in ascending order, convert it to a height balanced BST.


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


這裡兩道題目，是連在一起的兩題，給你一個排好序（升序）的數組或者鏈表，將它們轉為一棵平衡二叉樹。假設不排好序的話，一組隨機輸入的資料，就必須採用RBT或者AVL樹，這樣操作會變得更複雜，涉及到旋轉，但是這裡排好序了。所以，只要找到中位元，作為根，然後遞迴地根據中位元的左、右數列來構建左右子樹；


兩題的思路都是如上所述， 唯一的區別就是，鏈表尋找中位元會麻煩一些，需要引入fast、slow兩個指標，來尋找中位元，代碼如下：

1.Array

class Solution {public:    TreeNode *Tree(int left, int right, vector<int> &num){        TreeNode *root = NULL;        if (left <= right){            int cen = (left + right) / 2;            root = new TreeNode(num[cen]);            root->left = Tree(left, cen - 1, num);            root->right = Tree(cen + 1, right, num);        }        return root;    }        TreeNode *sortedArrayToBST(vector<int> &num) {        TreeNode *T = NULL;        int N = num.size();        T = Tree(0, N - 1, num);        return T;    }};

2.link-list

class Solution {public:    ListNode *findMid(ListNode *head){        //這裡如果鏈表中只有兩個數字，則mid返回的是head->next.        if (head == NULL || head -> next == NULL)            return head;        ListNode *fast, *slow, *pre;        fast = slow = head;        pre = NULL;        while (fast && fast->next){            pre = slow;            slow = slow->next;            fast = fast->next->next;        }        pre->next = NULL;        return slow;    }        TreeNode *buildTree(ListNode *head){        TreeNode *root = NULL;        ListNode *mid = NULL;        if (head){            mid = findMid(head);            root = new TreeNode(mid->val);            if (head != mid){                root->left = buildTree(head);                root->right = buildTree(mid->next);            }        }        return root;            }        TreeNode *sortedListToBST(ListNode *head) {        TreeNode *T;        T = buildTree(head);        return T;    }};

LeetCode :: Convert Sorted Array (link list) to Binary Search Tree [tree]