面试题13：有序单链表或者数组转平衡二叉搜索树

  • Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
  • Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 /**
10  * Definition for binary tree
11  * struct TreeNode {
12  *     int val;
13  *     TreeNode *left;
14  *     TreeNode *right;
15  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
16  * };
17  */
18 class Solution {
19 public:
20     TreeNode *sortedListToBST(ListNode *head) {
21         if(head == nullptr) return nullptr;
22 
23         ListNode* slow = head;
24         ListNode* fast = head;
25         ListNode* slow_pre = nullptr;
26         while(fast && fast->next){
27             slow_pre = slow;
28             slow = slow->next;
29             fast = fast->next->next;
30         }
31         if(slow_pre){
32             slow_pre->next = nullptr;
33         }
34         
35         TreeNode* root = new TreeNode(slow->val);
36         if(slow!= head)
37             root->left = sortedListToBST(head);
38         else
39             root->left = nullptr;
40         root->right = sortedListToBST(slow->next);
41 
42         return root;
43     }
44 };

 

/**
 * 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 *sortedArrayToBST(vector<int> &num) {
        int n = num.size();
        return sortedArrayToBST(num,0,n-1);
    }

    TreeNode* sortedArrayToBST(vector<int>& num,int l,int r){
        if(l > r) return nullptr;
        int mid = l + (r - l+1)/2;  //加1保证左子树节点个数不少于右边的节点个数，不加1也是平衡二叉树搜索树，但是无法通过验证
        TreeNode* root = new TreeNode(num[mid]);
        root->left = sortedArrayToBST(num,l,mid - 1);
        root->right = sortedArrayToBST(num,mid+1,r);
        return root;
    }

};

 

posted @ 2017-05-14 21:50  wxquare  阅读(691)  评论(0)    收藏  举报