Interview Classic 150 Questions P108 Convert Sorted Array to Binary Search Tree

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Timeline

Timeline

2025-12-01

init

Divide and Conquer, BST

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Problem:

P108 Convert Sorted Array to Binary Search Tree

https://leetcode.cn/problems/convert-sorted-array-to-binary-search-tree/description/?envType=study-plan-v2&envId=top-interview-150

Divide and Conquer approach:

#include <vector>
using std::vector;

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode()
		: val(0)
		, left(nullptr)
		, right(nullptr)
	{
	}
	TreeNode(int x)
		: val(x)
		, left(nullptr)
		, right(nullptr)
	{
	}
	TreeNode(int x, TreeNode *left, TreeNode *right)
		: val(x)
		, left(left)
		, right(right)
	{
	}
};

class Solution {
    private:
	TreeNode *buildBST(vector<int> &nums, int begin, int end)
	{
		if (end < begin) {
			return NULL;
		}
		// The inorder traversal result is a monotonically increasing sequence
		TreeNode *root = new TreeNode;
		int middle = (begin + end) / 2;
		root->val = nums[middle];
		root->left = buildBST(nums, begin, middle - 1);
		root->right = buildBST(nums, middle + 1, end);
		return root;
	}

    public:
	TreeNode *sortedArrayToBST(vector<int> &nums)
	{
		return buildBST(nums, 0, nums.size() - 1);
	}
};

leetcode hot 100 rewrite

struct TreeNode {
        int val;
        TreeNode *left;
        TreeNode *right;
        TreeNode()
                : val(0)
                , left(nullptr)
                , right(nullptr)
        {
        }
        TreeNode(int x)
                : val(x)
                , left(nullptr)
                , right(nullptr)
        {
        }
        TreeNode(int x, TreeNode *left, TreeNode *right)
                : val(x)
                , left(left)
                , right(right)
        {
        }
};
#include <vector>
using std::vector;

class Solution {
    private:
        TreeNode *buildBST(vector<int> &nums, int start, int end)
        {
                if (start > end)
                        return nullptr;
                int mid = (start + end) / 2;

                TreeNode *root = new TreeNode(nums[mid]);
                root->left = buildBST(nums, start, mid - 1);
                root->right = buildBST(nums, mid + 1, end);
                return root;
        }

    public:
        TreeNode *sortedArrayToBST(vector<int> &nums)
        {
                return buildBST(nums, 0, nums.size() - 1);
        }
};