Max Tree

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     /**
14      * @param A: Given an integer array with no duplicates.
15      * @return: The root of max tree.
16      */
17     public TreeNode maxTree(int[] A) {
18         int size = A.length;
19         if(A == null || size == 0) {
20             return null;
21         }
22         
23         Stack<TreeNode> stack = new Stack<TreeNode>();
24         for(int i = 0; i <= size; i++) {
25             TreeNode right = i == size ? new TreeNode(Integer.MAX_VALUE) 
26                 : new TreeNode(A[i]);
27                 
28             while(!stack.isEmpty() && right.val > stack.peek().val) {
29                 TreeNode node = stack.pop();
30                 if(stack.isEmpty()) {
31                     right.left = node;
32                 }else {
33                     TreeNode left = stack.peek();
34                     if(left.val > right.val) {
35                         right.left = node;
36                     }else {
37                         left.right = node;
38                     }
39                 }
40             }
41             
42             stack.push(right);
43         }
44         
45         return stack.peek().left;
46     }
47 }