【leetcode】Convert Sorted Array to Binary Search Tree
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一、问题描述
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
二、问题分析
先复习下什么是二叉搜索树(引自Wikipedia):
二叉查找树(Binary Search Tree),也称有序二叉树(ordered binary tree),排序二叉树(sorted binary tree),是指一棵空树或者具有下列性质的二叉树:
若任意节点的左子树不空,则左子树上所有结点的值均小于它的根结点的值;
任意节点的右子树不空,则右子树上所有结点的值均大于它的根结点的值;
任意节点的左、右子树也分别为二叉查找树。
然后就是树的创建过程,其实树的创建和树的遍历非常相似,往往是采用递归的方式建立。再分析题目,给定数组是排好序的,结合BST的特点,显然是采用二分法,然后递归建立左右子树。
三、Java AC 代码
public TreeNode sortedArrayToBST(int[] nums) {TreeNode root = null;if (nums == null || nums.length == 0) {return root;}int start = 0, end = nums.length - 1;root = createTree(root, start, end, nums);return root;}public TreeNode createTree(TreeNode node, int start, int end, int[] nums) {if (start > end) {return null;}int mid = (start + end) / 2;node = new TreeNode(nums[mid]);node.left = createTree(node.left, start, mid - 1, nums);node.right = createTree(node.right, mid + 1, end, nums);return node;}
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