java实现——006重建二叉树

 public class T006 {
     public static void main(String[] args){
         int pre[] = {1,2,4,7,3,5,6,8};
         int in[] = {4,7,2,1,5,3,8,6};
         preorderTraversalRec(construct(pre,in,8));

     }
     public static void preorderTraversalRec(TreeNode root) {
         if (root == null) {
             return;
         }
         System.out.print(root.val + " ");
         preorderTraversalRec(root.left);
         preorderTraversalRec(root.right);
     }
     public static TreeNode construct(int[] pre,int[] in,int length){
         if(pre == null ||in == null||length<=0)
             return null;
         return constructCore(pre,in,0,length-1,0,length-1);

     }
     public static TreeNode constructCore(int[] pre,int[] in,int startPre,int endPre,int startIn,int endIn){
         int rootValue = pre[startPre];
         TreeNode root = new TreeNode(rootValue);
         root.left = null;
         root.right = null;
         if(startPre == endPre){
             if(startIn == endIn&&startPre==startIn){
                 System.out.println("root");
                 return root;
             }
         }
         int rootIn = startIn;
         while(rootIn<=endIn&&in[rootIn]!=rootValue){
             ++ rootIn;
         }
         if(rootIn == endIn&&in[rootIn]!=rootValue){
             System.out.println("Invalid input2");
         }
         int leftLength = rootIn - startIn;
         int leftPreEnd = startPre + leftLength;
         if(leftLength>0){
             root.left = constructCore(pre,in,startPre+1,leftPreEnd,startIn,rootIn-1);
         }
         if(leftLength<endPre-startPre){
             root.right = constructCore(pre,in,leftPreEnd+1,endPre,rootIn+1,endIn);
         }
         return root;

     }
     private static class TreeNode {
         int val;
         TreeNode left;
         TreeNode right;

         public TreeNode(int val) {
             this.val = val;
         }
     }
 }