108. Convert Sorted Array to Binary Search Tree

ConvertSortedArrayToBinarySearchTree/step1.md

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+```java
+// Thought it will be a binary search.
+// Just use the way like binary search, put the mid node as parent
+// Then put left and right side will be the children node of the parent.
+// Repeat the process and we will have the tree.
+// Time complexity: O(N) N: length of the given array
+// Space complexity: O(N)
+// Time spend: N/A (Checked Solution tab)
+class Solution {
+    public TreeNode sortedArrayToBST(int[] nums) {
+         return sortedArrayToBST(nums, 0, nums.length - 1);
+    }
+
+    public TreeNode sortedArrayToBST(int[] nums, int start, int end) {
+        if (start > end) {
+            return null;
+        }
+
+        int mid = (start + end) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = sortedArrayToBST(nums, start, mid - 1);
+        root.right = sortedArrayToBST(nums, mid + 1, end);
+
+        return root;
+    }
+
+    public TreeNode sortedArrayToBST(int[] nums) {
+        return sortedArrayToBST(nums, 0, nums.length);
+    }
+
+    public TreeNode sortedArrayToBST(int[] nums, int start, int end) {
+        if (start >= end) {
+            return null;
+        }
+
+        int mid = (start + end) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = sortedArrayToBST(nums, start, mid);
+        root.right = sortedArrayToBST(nums, mid + 1, end);
+
+        return root;
+    }
+}
+```

ConvertSortedArrayToBinarySearchTree/step2.md

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+```java
+class Solution {
+    public TreeNode sortedArrayToBST(int[] nums) {
+         return convertSortedArrayToBST(nums, 0, nums.length - 1);
+    }
+
+    public TreeNode convertSortedArrayToBST(int[] nums, int start, int end) {
+        if (start > end) {
+            return null;
+        }
+
+        if (start == end) {
+            return new TreeNode(nums[start]);
+        }
+
+        int mid = (start + end) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = convertSortedArrayToBST(nums, start, mid - 1);
+        root.right = convertSortedArrayToBST(nums, mid + 1, end);
+
+        return root;
+    }
+}
+```

ConvertSortedArrayToBinarySearchTree/step3.md

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+```java
+// Time spend: 02:31
+class Solution {
+    public TreeNode sortedArrayToBST(int[] nums) {
+        return convertSortedArrayToBst(nums, 0, nums.length - 1);
+    }
+
+    public TreeNode convertSortedArrayToBst(int[] nums, int l, int r) {
+        if (l > r) {
+            return null;
+        }
+
+        if (l == r) {
+            return new TreeNode(nums[l]);
+        }
+
+        int mid = (l + r) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = convertSortedArrayToBst(nums, l, mid - 1);
+        root.right = convertSortedArrayToBst(nums, mid + 1, r);
+
+        return root;
+    }
+}
+```
+
+```java
+// Time spend: 01:42
+class Solution {
+    public TreeNode sortedArrayToBST(int[] nums) {
+        return convertSortedArrayToBst(nums, 0, nums.length - 1);
+    }
+
+    public TreeNode convertSortedArrayToBst(int[] nums, int l, int r) {
+        if (l > r) {
+            return null;
+        }
+        if (l == r) {
+            return new TreeNode(nums[l]);
+        }
+
+        int mid = (l + r) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = convertSortedArrayToBst(nums, l, mid - 1);
+        root.right = convertSortedArrayToBst(nums, mid + 1, r);
+
+        return root;
+    }
+}
+```
+
+```java
+// Time spend: 01:42
+class Solution {
+    public TreeNode sortedArrayToBST(int[] nums) {
+        return convertSortedArrayToBst(nums, 0, nums.length - 1);
+    }
+
+    public TreeNode convertSortedArrayToBst(int[] nums, int l, int r) {
+        if (l > r) {
+            return null;
+        }
+
+        if (l == r) {
+            return new TreeNode(nums[l]);
+        }
+
+        int mid = (l + r) / 2;
+        TreeNode root = new TreeNode(nums[mid]);
+        root.left = convertSortedArrayToBst(nums, l, mid - 1);
+        root.right = convertSortedArrayToBst(nums, mid + 1, r);
+
+        return root;
+    }
+}
+```