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108. Convert Sorted Array to Binary Search Tree 👍

  • Time: $O(n)$
  • Space: $O(\log n)$
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class Solution {
 public:
  TreeNode* sortedArrayToBST(vector<int>& nums) {
    return build(nums, 0, nums.size() - 1);
  }

 private:
  TreeNode* build(const vector<int>& nums, int l, int r) {
    if (l > r)
      return nullptr;
    const int m = (l + r) / 2;
    return new TreeNode(nums[m], build(nums, l, m - 1), build(nums, m + 1, r));
  }
};

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class Solution {
  public TreeNode sortedArrayToBST(int[] nums) {
    return build(nums, 0, nums.length - 1);
  }

  private TreeNode build(int[] nums, int l, int r) {
    if (l > r)
      return null;
    final int m = (l + r) / 2;
    return new TreeNode(nums[m], build(nums, l, m - 1), build(nums, m + 1, r));
  }
}

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class Solution:
  def sortedArrayToBST(self, nums: list[int]) -> TreeNode | None:
    def build(l: int, r: int) -> TreeNode | None:
      if l > r:
        return None
      m = (l + r) // 2
      return TreeNode(nums[m],
                      build(l, m - 1),
                      build(m + 1, r))

    return build(0, len(nums) - 1)