[Leetcode][JAVA] Convert Sorted Array to Binary Search Tree && Convert Sorted List to Binary Search Tree

標籤：

Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

很簡單的二分法，只要給出Array的開始和結束下標作為參數傳入即可。

 1  public TreeNode sortedArrayToBST(int[] num) { 2         return constructBST(num,0,num.length-1); 3     } 4     public TreeNode constructBST(int[] num, int start, int end) { 5         if(start>end) 6             return null; 7         if(start==end) 8             return new TreeNode(num[start]); 9         int mid = (start+end)/2;10         TreeNode root = new TreeNode(num[mid]);11         root.left = constructBST(num, start, mid-1);12         root.right = constructBST(num, mid+1, end);13         return root;14     }

Convert Sorted List to Binary Search Tree

 Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

對於鏈表來說，情況稍微複雜一些，最簡單的思路是可以遍曆一遍鏈表存到array中然後構建。這裡採用直接構建的方法，就是用快慢指標找到鏈表中點，左半部分鏈表終點指向null，遞迴構建成左子樹。右半部分遞迴構建成右子樹即可。如果不想破壞原有鏈表那麼可以採用先存到array中再構建的方法。

 1     public TreeNode sortedListToBST(ListNode head) { 2         if(head==null) 3             return null; 4         if(head.next == null) 5             return new TreeNode(head.val); 6         ListNode fast = head.next; 7         ListNode slow = head; 8         while(fast!=null && fast.next!=null && fast.next.next!=null) { 9             fast = fast.next.next;10             slow = slow.next;11         }12         ListNode root = slow.next;13         slow.next = null;14         TreeNode tRoot = new TreeNode(root.val);15         tRoot.left = sortedListToBST(head);16         tRoot.right = sortedListToBST(root.next);17         return tRoot;18     }

 

[Leetcode][JAVA] Convert Sorted Array to Binary Search Tree && Convert Sorted List to Binary Search Tree