【Leetcode长征系列】Convert Sorted List to Binary Search Tree

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。通过递归方法实现，并详细展示了使用C++代码的具体实现过程。

原题：

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

思路：和处理数组的思想类似，用递归。

代码：

/**
 * 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) {
        if(head==NULL) return NULL;
        ListNode *tmp = head;
        vector<ListNode *> list;
        while(!tmp){
            list.push_back(tmp);
            tmp = tmp->next;
        }
        return buildBST(list, 0, list.size()-1);
    }
    
    TreeNode *buildBST(vector<ListNode *> &list, int left, int right){
        if(left>right) return NULL;
        int mid = (left+right)/2;
        TreeNode *root = new TreeNode(list[mid]->val);
        root->left = buildBST(list, left, mid-1);
        root->right = buildBST(list, mid+1, right);
        return root;
    }
};

确实不懂为什么返回值永远是空串。

/**
 * 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) {
        if(!head) return NULL;
        TreeNode *root = NULL;
        ListNode *tmp = head;
        vector<int> list;
        while(!tmp){
            list.push_back(tmp->val);
            tmp = tmp->next;
        }
    
        buildBST(list, root, 0, list.size()-1);
        return root;
    }
    
    void buildBST(vector<int> &list, TreeNode *&root, int left, int right){
        if(left>right) return;
        int mid = (left+right)/2;
        root = new TreeNode(list[mid]);
        buildBST(list, root->left, left, mid-1);
        buildBST(list, root->right, mid+1, right);
    }
};

while(tmp)....

以后没事还是直接用！=NULL来表示好了……