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Author: BaffinLee

101-200/108. Convert Sorted Array to Binary Search Tree.md

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+# 108. Convert Sorted Array to Binary Search Tree
+
+- Difficulty: Easy.
+- Related Topics: Tree, Depth-first Search.
+- Similar Questions: Convert Sorted List to Binary Search Tree.
+
+## Problem
+
+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
+
+```javascript
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {number[]} nums
+ * @return {TreeNode}
+ */
+var sortedArrayToBST = function(nums) {
+  return helper(nums, 0, nums.length - 1);
+};
+
+var helper = function (nums, ll, rr) {
+  if (ll > rr) return null;
+  var mid = Math.ceil((ll + rr) / 2);
+  var root = new TreeNode(nums[mid]);
+  root.left = helper(nums, ll, mid - 1);
+  root.right = helper(nums, mid + 1, rr);
+  return root;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).