重建二叉树

Posted 2020-08-09 Gump Yan

题目描述

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode reConstructBinaryTree(int [] pre,int [] in) {
        return reConBTree(pre,0,pre.length-1,in,0,in.length-1);
    }
    public TreeNode reConBTree(int [] pre,int preleft,int preright,int [] in,int inleft,int inright){
        if(preleft > preright || inleft> inright)//当到达边界条件时候返回null
            return null;
        //新建一个TreeNode
        TreeNode root = new TreeNode(pre[preleft]);//前序数组第一个元素就是根节点
        //对中序数组进行输入边界的遍历
        for(int i = inleft; i<= inright; i++){ 
            if(pre[preleft] == in[i]){
                //重构左子树，注意边界条件
                root.left = reConBTree(pre,preleft+1,preleft+i-inleft,in,inleft,i-1);
                //重构右子树，注意边界条件
                root.right = reConBTree(pre,preleft+i+1-inleft,preright,in,i+1,inright);
            }
        }
        return root;     
    }
}