重建二叉树

 1 public class Solution {
 2     public TreeNode reConstructBinaryTree(int [] pre, int [] in) {
 3         TreeNode  root = help(pre,in,0,pre.length-1,0,in.length-1);
 4         return root;
 5     }
 6     private TreeNode help(int[] pre, int[] in,int prebeg,int preend,int inbeg,int inend ){
 7         if(prebeg>preend||inbeg>inend)
 8             return null;
 9         TreeNode root = new TreeNode(pre[prebeg]);
10         int index = 0;
11         for(int i=0;i<in.length;i++){   
12             if(in[i]==pre[prebeg]){
13                 index = i;
14                 break;
15             }
16         }
17         root.left = help(pre,in,prebeg+1,prebeg+(index-inbeg),inbeg,index-1);
18         root.right = help(pre,in,prebeg+(index-inbeg)+1,preend,index+1,inend);
19         return root;
20     }
21 }