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.

要求高度平衡，那么应该尽量把元素往左右子树平均分配。正好又是有序数组，所以可以把数组分开，中间元素作为根，左边作为左子树，右边右子树。

public TreeNode sortedArrayToBST(int[] nums){int len=nums.length;if(len==0)return null;return build(0, len-1, nums);}private TreeNode build(int start,int end,int[] nums){if(start==end)return new TreeNode(nums[start]);if(start>end)return null;int mid=(start+end)>>1;int num=nums[mid];TreeNode t=new TreeNode(num);t.left=build(start, mid-1, nums);t.right=build(mid+1, end, nums);return t;}