Return the root node of a binary search tree that matches the given preorder
traversal.
(Recall that a binary search tree is a binary tree where for every node, any descendant of node.left
has a value <
node.val
, and any descendant of node.right
has a value >
node.val
. Also recall that a preorder traversal displays the value of the node
first, then traverses node.left
, then traverses node.right
.)
It's guaranteed that for the given test cases there is always possible to find a binary search tree with the given requirements.
Example 1:
Input: [8,5,1,7,10,12]
Output: [8,5,10,1,7,null,12]
Constraints:
1 <= preorder.length <= 100
1 <= preorder[i] <= 10^8
- The values of
preorder
are distinct.
class Solution { public TreeNode bstFromPreorder(int[] preorder) { if (preorder == null || preorder.length == 0) { return null; } Stack<TreeNode> stack = new Stack<>(); TreeNode root = new TreeNode(preorder[0]); stack.push(root); for (int i = 1; i < preorder.length; i++) { TreeNode node = new TreeNode(preorder[i]); if (preorder[i] < stack.peek().val) { stack.peek().left = node; } else { TreeNode parent = stack.peek(); while (!stack.isEmpty() && preorder[i] > stack.peek().val) { parent = stack.pop(); } parent.right = node; } stack.push(node); } return root; } }