convert-sorted-list-to-binary-search-tree
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题目描述
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
从给定的有序链表生成平衡二叉树。
解题思路:最容易想到的就是利用数组生成二叉树的方法,找到中间节点作为二叉树的root节点,然后分别对左右链表递归调用分别生成左子树和右子树。时间复杂度O(N*lgN)
代码1:
public class Solution { ListNode getLeftNodeFromList(ListNode head) { ListNode next = head; ListNode current = head; ListNode pre = head; while(next!=null) { next = next.next; if(next==null) { break; } next = next.next; if(next==null) { break; } pre = head; head = head.next; } return pre; } public TreeNode sortedListToBST(ListNode head) { if(head==null) { return null; } if(head.next==null) { return new TreeNode(head.val); } ListNode left = getLeftNodeFromList(head); ListNode mid = left.next; TreeNode root = new TreeNode(mid.val); left.next = null; root.left = sortedListToBST(head); root.right = sortedListToBST(mid.next); return root; }}
网上又看到一种自底向上的方法,算法复杂度为O(N)。先递归构建左子树,在构建左子树的同时不断移动链表的头指针,链表的头指针永远是对应当前子树位置的。一直到左叶子节点,左叶子节点对应的就是链表的第一个元素,生成左叶子节点之后移动链表当前指针。
代码2:
public class Solution { static ListNode currentHead = null; TreeNode buildTree(int start, int end) { if(start>end) { return null; } int mid = start + (end - start)/2; TreeNode left = buildTree(start, mid-1); TreeNode root = new TreeNode(currentHead.val); root.left = left; currentHead = currentHead.next; root.right = buildTree(mid + 1, end); return root; } public TreeNode sortedListToBST(ListNode head) { if(head==null) { return null; } currentHead = head; int len = 0; while(head!=null) { len++; head = head.next; } return buildTree(0, len-1); }}
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- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- convert sorted list to binary search tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
- Convert Sorted List to Binary Search Tree
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