LC.108.Convert Sorted Array to Binary Search Tree

Posted 2020-10-26 davidnyc

https://leetcode.com/problems/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


time: o(n) space: o(height)
pre order: 先弄自己， 再管下面的兄弟

public TreeNode sortedArrayToBST(int[] nums) {
        if (nums  == null || nums.length ==0 )return null ;
        int left = 0, right = nums.length -1 ;
        return helper(nums, left, right);
    }

    //everytime create the root and connect to left and right
    private TreeNode helper(int[] nums, int left, int right) {
        int mid = left + (right - left) /2 ;
        TreeNode node = new TreeNode(nums[mid]) ;
        if (mid+ 1<=right){
            node.right = helper(nums, mid+1, right) ;
        }
        if (mid -1 >= left){
            node.left = helper(nums, left, mid-1) ;
        }
        //going down and repeat
        return node ;
    }