[LeetCode] Convert Sorted List to Binary Search Tree

最新推荐文章于 2022-02-19 22:56:38 发布

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本文介绍如何将一个按升序排列的单链表转换为高度平衡的二叉搜索树。通过将链表元素存储到数组中，再利用递归方法构建平衡二叉搜索树，最终实现链表到平衡二叉树的转换。

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

[Analysis]
和 Convert Sorted Array to Binary Search Tree 差不多，将链表中的数值用一个vector存就转换成上一题了。

[Solution]

//
// 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:
    // build tree
    TreeNode *build(vector &num, int begin, int end){
        if(begin > end){
            return NULL;
        }
   
        // build left brunch and right brunch
        int mid = (begin+end)/2;
        TreeNode* left = build(num, begin, mid-1);
        TreeNode* right = build(num, mid+1, end);
   
        // build tree
        TreeNode* node = new TreeNode(num[mid]);
        node->left = left;
        node->right = right;
        return node;
    }
   
    // sorted array to BST
    TreeNode *sortedArrayToBST(vector &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(num.size() == 0){
            return NULL;
        }
        else{
            return build(num, 0, num.size()-1);
        }
    }
   
    // sorted list to BST
    TreeNode *sortedListToBST(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector array;
        while(head != NULL){
            array.push_back(head->val);
            head = head->next;
        }
        return sortedArrayToBST(array);
    }
};