LeetCode Convert Sorted List to Binary Search Tree

coobj2008

于 2014-03-18 20:59:03 发布

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本文介绍了一种将升序排列的单链表转换为高度平衡的二叉搜索树的方法。通过递归地选取中间节点作为根节点，并以此为基础构建左右子树，最终实现从链表到平衡二叉搜索树的转换。

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题目：

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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        int cnt = 0;
        ListNode *cur = head;
        while(cur != NULL) {
            cnt++;
            cur= cur->next;
        }
        TreeNode *root = create(head, 0, cnt-1);
        return root;
    }
private:
    //自底向上构造树，左边构造完时，创建root，然后构造右子树
    TreeNode *create(ListNode *&head, int begin, int end) {
        if(begin > end)
            return NULL;
        int mid = begin + (end-begin)/2;
        TreeNode *left = create(head, begin, mid-1);
        TreeNode *root = new TreeNode(head->val);
        root->left = left;
        head = head->next;
        TreeNode *right = create(head, mid+1, end);
        root->right = right;
        return root;
    }
};