LeetCode: 108. Convert Sorted Array to Binary Search Tree

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108. Convert Sorted Array to Binary Search Tree

LeetCode - 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

解题分析

这道题跟 109. Convert Sorted List to Binary Search Tree 类似。

代码还可以再简化一下。

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/**
 * Runtime: 0 ms, faster than 100.00% of Java online submissions for Convert Sorted Array to Binary Search Tree.
 *
 * Memory Usage: 47 MB, less than 5.16% of Java online submissions for Convert Sorted Array to Binary Search Tree.
 */
public TreeNode sortedArrayToBST(int[] nums) {
    if (Objects.isNull(nums) || nums.length == 0) {
        return null;
    }
    int high = nums.length;
    int mid = high / 2;
    TreeNode root = new TreeNode(nums[mid]);
    build(root, true, nums, 0, mid - 1);
    build(root, false, nums, mid + 1, high - 1);
    return root;
}

private void build(TreeNode root, boolean isLeft, int[] nums, int low, int high) {
    if (low > high) {
        return;
    }
    int mid = low + (high - low) / 2;
    TreeNode node = new TreeNode(nums[mid]);
    if (isLeft) {
        root.left = node;
    } else {
        root.right = node;
    }
    build(node, true, nums, low, mid - 1);
    build(node, false, nums, mid + 1, high);
}