108. Convert Sorted Array to Binary Search Tree
Tags: ‘Tree’, ‘Depth-first Search’
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.
Example:
Given the sorted array: [-10,-3,0,5,9],
One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:
0
/ \
-3 9
/ /
-10 5
Solution
方法1： DFS 答案不唯一，当偶数个时取左还是取右 O(n) space O(n) time
public TreeNode sortedArrayToBST(int[] nums) {
return helper(nums, 0, nums.length - 1);
}
private TreeNode helper(int[] nums, int l, int r) {
if (l > r) {
return null;
}
int m = l + (r - l) / 2;
TreeNode root = new TreeNode(nums[m]);
root.left = helper(nums, l, m - 1);
root.right = helper(nums, m + 1, r);
return root;
}