108. Convert Sorted Array to Binary Search Tree 109. Convert Sorted List to Binary Search Tree -- 将有序数组或有序链表转成平衡二叉排序树

108. Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(vector<int> nums, int left, int right)
    {
        if(left > right)
            return NULL;
        TreeNode *node;
        if(left == right)
        {
            node = new TreeNode(nums[left]);
            return node;
        }
        int mid = (left+right)/2;
        node = new TreeNode(nums[mid]);
        node->left = buildTree(nums, left, mid-1);
        node->right = buildTree(nums, mid+1, right);
        return node;
    }
    
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        int n = nums.size();
        if(!n)
            return NULL;
        TreeNode *root = buildTree(nums, 0, n-1);
        return root;
    }
};

109. Convert Sorted List to Binary Search Tree

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 a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(ListNode* &head, int left, int right)
    {
        if(left > right)
            return NULL;
        int mid = (left + right) / 2;
        TreeNode *leftNode = buildTree(head, left, mid-1);
        TreeNode *node = new TreeNode(head->val);
        node->left = leftNode;
        head = head->next;
        TreeNode *rightNode = buildTree(head, mid+1, right);
        node->right = rightNode;
        return node;
    }
    
    TreeNode* sortedListToBST(ListNode* head) {
        int len = 0;
        for(ListNode *p = head; p; p = p->next)
            len++;
        if(!len)
            return NULL;
        TreeNode *root = buildTree(head, 0, len-1);
        return root;
    }
};

先建左子树，然后head后移，再建右子树。