LeetCode 108 Convert Sorted Array to Binary Search Tree

题意

给予一个从小到大的数组, 构建一颗二叉平衡树, 即每个节点的两个子树的深度不能相差超过 1.

例 :

给予数组: [-10, -3, 0, 5, 9]

返回以下树都是符合条件的: 
      0
     / \
   -3   9
   /   /
 -10  5


      0
    /  \
  -10   5
    \    \
    -3    9

解法

这道题有点类似于二分查找法, 即对二叉搜索树的反向推导, 每次取数组最中间的节点最为中节点, 左侧为左子树的内容, 右侧为右子树的内容, 直到不再存在子树.

import util.TreeNode;

/**
 * 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 helper(nums, 0, nums.length - 1);
    }

    private TreeNode helper(int[] nums, int start, int end) {
        if (start > end) {
            return null;
        }

        int mid = (start + end) / 2;
        TreeNode node = new TreeNode(nums[mid]);
        node.left = helper(nums, start, mid - 1);
        node.right = helper(nums, mid + 1, end);
        return node;
    }
}

Runtime: 0 ms, faster than 100.00% of Java online submissions for Convert Sorted Array to Binary Search Tree.
Memory Usage: 35.8 MB, less than 99.88% of Java online submissions for Convert Sorted Array to Binary