34. Convert Sorted List to Binary Search Tree && Convert Sorted Array to Binary Search Tree

Convert Sorted List to Binary Search Tree

OJ: https://oj.leetcode.com/problems/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.

思想： 以中间点为根节点，按先序顺序来创建 。

/**
 * 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) {}
 * };
 */
ListNode* findMiddleNode(ListNode *head, int length) {
    if(length <= 0) return NULL;
    int mid = (length+1) / 2;
    for(int i = 1; i < mid; ++i) head = head->next;
    return head;
}
TreeNode* createBST(ListNode *head, int length) {
    ListNode *pMid = findMiddleNode(head, length);
    TreeNode *root = NULL;
    if(pMid) {
        root = new TreeNode(pMid->val);
        root->left = createBST(head, (length-1) >> 1);
        root->right = createBST(pMid->next, length / 2);
    }
    return root;
}
int getLength(ListNode *head) {
    int len = 0;
    while(head) { len++; head = head->next; }
    return len;
}
class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        return createBST(head, getLength(head));
    }
};

Convert Sorted Array to Binary Search Tree

OJ: https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

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

思想，同上。

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