Leetcode 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.

method 1

本題只要熟悉概念就會做，構造的二叉樹要求是AVL樹，那麼我們可以使用分治法，取中間的數作爲根節點，然後對左右兩邊採取同樣的操作

TreeNode* helper(vector<int>& nums, int low, int high){
	if (low > high) return NULL;

	int mid = (low + high) / 2;
	TreeNode* root = new TreeNode(nums[mid]);
	root->left = helper(nums, low, mid - 1);
	root->right = helper(nums, mid + 1, high);
	return root;
}

TreeNode* sortedArrayToBST(vector<int>& nums) {
	if (nums.size() == 0)
		return NULL;

	return helper(nums, 0, nums.size() - 1);
}

summary

1. 大問題可以化爲小問題–> 分治法