重建二叉树

来源：互联网发布：淘宝店铺新手装修步骤编辑：程序博客网时间：2024/06/15 02:15

已知前序和中序，重佳二叉树，以层序输出：

import java.util.LinkedList;import java.util.Queue;/** * Created by wqh on 2017/4/23. */public class Num_9 {    public TreeNode reBuildHelper(int[] preOrder, int[] inOrder, int n){        if(n<=0 || preOrder==null || inOrder==null)            return null;        return reBuild(preOrder, 0, preOrder.length-1, inOrder, 0,inOrder.length-1);    }    public TreeNode reBuild(int[] preOrder, int preStart, int preEnd, int[] inOrder, int inStart, int inEnd){        int beginVal = preOrder[preStart];        TreeNode root = new TreeNode(beginVal);        //当只有一个节点时        if(preStart==preEnd)            if(preOrder[preStart] == inOrder[inStart])                return root;            else throw new RuntimeException("preOder is not same with inOrder");        int rootIndex = inStart;        //遍历中序，得到根节点所在索引        while(rootIndex<=preEnd && inOrder[rootIndex]!=beginVal)            rootIndex++;        int leftLen = rootIndex - inStart;        //构建左子树        if(leftLen > 0)            root.left = reBuild(preOrder,preStart+1,preStart+leftLen, inOrder, inStart, rootIndex-1);        //构建右子树        if(inEnd > rootIndex)            root.right = reBuild(preOrder, preStart+leftLen+1, preEnd, inOrder, rootIndex+1, inEnd);        return root;    }    //层序遍历（采用队列的思想）    public void levelOrder(TreeNode root){        Queue<TreeNode> queue = new LinkedList<>();        queue.add(root);        while(!queue.isEmpty()){            TreeNode node = queue.poll();            System.out.print(node.val+" ");            if(node.left != null) queue.add(node.left);            if(node.right != null) queue.add(node.right);        }    }    public static void main(String[] args){        int[] preOrder = new int[]{1,2,4,7,3,5,6,8};        int[] inOrder = new int[]{4,7,2,1,5,3,8,6};        Num_9 test = new Num_9();        TreeNode root = test.reBuildHelper(preOrder, inOrder, preOrder.length);        test.levelOrder(root);    }}

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