1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public TreeNode buildTree(int[] preorder, int[] inorder) {
12         int length = inorder.length;
13         return buildTreeHelp(preorder,inorder,0,length - 1, 0, length - 1);
14     }
15     
16     public TreeNode buildTreeHelp(int[] preorder, int[] inorder, int pstart, int pend, int istart,int iend){
17         TreeNode root = null;
18         if(istart <= iend){
19             root = new TreeNode(preorder[pstart]);
20             int rootPosition = istart;
21             int newpend = pstart;
22             for(int i = istart; i <= iend; ++i){
23                 if(inorder[i] == preorder[pstart]){
24                     rootPosition = i;
25                     break;
26                 }
27                 else
28                     ++newpend;                
29             }
30             root.left = buildTreeHelp(preorder, inorder, pstart + 1, newpend, istart, rootPosition - 1);
31             root.right = buildTreeHelp(preorder, inorder, newpend + 1, pend, rootPosition + 1, iend);
32         }
33         return root;
34     }
35 }