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  • 108. Convert Sorted Array to Binary Search Tree

    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

    注意edge case: start > end的时候返回null

    time: O(n), space: O(n)  -- O(logn) for recursive call, and O(n) for constructing BST

    /**
     * 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) {
            if(nums == null || nums.length == 0) return null;
            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 - start) / 2;
            TreeNode root = new TreeNode(nums[mid]);
            root.left = helper(nums, start, mid - 1);
            root.right = helper(nums, mid + 1, end);
            return root;
        }
    }
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  • 原文地址:https://www.cnblogs.com/fatttcat/p/10145175.html
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