24. Construct_Binary_Search_Tree_from_Preorder_Traversal.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int preIndex =0;
    public TreeNode bstFromPreorder(int[] preorder) {
        return constructBST(preorder,Integer.MAX_VALUE); 
    }

    TreeNode constructBST(int[] preOrder,int val){
        if (preIndex==preOrder.length || val<preOrder[preIndex])
            return null;
        TreeNode root=new TreeNode(preOrder[preIndex++]);
        root.left=constructBST(preOrder,root.val);
        root.right=constructBST(preOrder,val);
        return root;
    }
}