[LeetCode] Convert Sorted List to Binary Search Tree

zjjee

于 2014-12-25 13:36:34 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。通过计算链表长度并递归地选择中间元素作为根节点，确保了树的高度平衡性。

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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:
    int length(ListNode *p){
        int cnt = 0;
        while(p){
            cnt ++;
            p = p -> next;
        }
        return cnt;
    }
    TreeNode *generate(ListNode *node,int left,int right){
        if(left > right) return NULL;
        int mid = (left + right) / 2;
        ListNode *p = node;
        for(int i = left;i < mid;i ++)
            p = p -> next;
        TreeNode *tleft = generate(node,left,mid - 1);
        TreeNode *tright = generate(p -> next,mid + 1,right);
        TreeNode *root = new TreeNode(p -> val);
        root -> left = tleft;
        root -> right = tright;
        return root;
    }
    TreeNode *sortedListToBST(ListNode *head) {
        int len = length(head);
        return generate(head,0,len - 1);
    }
};