ConvertSortedArraytoBinarySearchTree

Change-Id: Ie9080e731479150c99c89ccd69bf3136212b4167

Author: applewjg

ConvertSortedArraytoBinarySearchTree.java

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+/*
+ Author:     Andy, [email protected]
+ Date:       Jan 29, 2015
+ Problem:    Convert Sorted Array to Binary Search Tree
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/
+ Notes:
+ Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
+
+ Solution: Recursion.
+ */
+/**
+ * Definition for binary tree
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+public class Solution {
+    public TreeNode sortedArrayToBST(int[] num) {
+        return sortedArrayToBSTRe(num, 0, num.length - 1);
+    }
+    public TreeNode sortedArrayToBSTRe(int[] num, int left, int right) {
+        if (left > right) return null;
+        if (left == right) return new TreeNode(num[left]);
+        int mid = (left + right) / 2;
+        TreeNode node = new TreeNode(num[mid]);
+        node.left = sortedArrayToBSTRe(num, left, mid - 1);
+        node.right = sortedArrayToBSTRe(num, mid + 1, right);
+        return node;
+    }
+}