[LeetCode-23] Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

Because the requirement "height balanced", this problem becomes relative easy.

Just recursively use the middle element in the array as the root node of the current tree(sub tree).

c++

TreeNode *buildArrayBST(vector<int> &num, int start, int end){
    if(start>end) return NULL;
    int mid = (start+end)/2;
    TreeNode *root = new TreeNode(num[mid]);
    root->left = buildArrayBST(num,start,mid-1);
    root->right = buildArrayBST(num,mid+1,end);
    return root;
}
TreeNode *sortedArrayToBST(vector<int> &num) {
    return buildArrayBST(num,0,num.size()-1);
}

java

public TreeNode sortedArrayToBST(int[] num) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        return BuildTree(num, 0, num.length-1);
    }
	public TreeNode BuildTree(int[] num, int start, int end){
		if(start>end)
			return null;
		int mid = (start+end)/2;
		TreeNode node = new TreeNode(num[mid]);
		node.left = BuildTree(num, start, mid-1);
		node.right = BuildTree(num, mid+1, end);
		return node;
	}