Convert Sorted Array to Binary Search Tree

Description

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

思路

有序数组的中间作为平衡二叉树的根节点，前半部分作为左子树，后半部分作为右子树。

实现代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public TreeNode sortedArrayToBST(int[] nums) {
    return BST(nums, 0, nums.length - 1);
  }

  public TreeNode BST(int[] nums, int start, int end) {
    if (start > end) return null;
    int mid = (start + end) / 2;
    TreeNode root = new TreeNode(nums[mid]);
    root.left = BST(nums, start, mid - 1);
    root.right = BST(nums, mid + 1, end);
    return root;
  }
}