20200825.md

Algorithm

剑指offer-重建二叉树

Description

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。

Solution

/**
 * 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) {
        if(pre==null || in ==null){
          return null;
        }
        if(pre.length==0||in.length==0){
          return null;
        }
        TreeNode root = null;
        for(int i=0;i<in.length;i++){
           if(pre[0] == in[i]){
              root = new TreeNode(in[i]);
              root.left = reConstructBinaryTree(Arrays.copyOfRange(pre, 1, i+1),Arrays.copyOfRange(in, 0, i));
              root.right = reConstructBinaryTree(Arrays.copyOfRange(pre, i+1, pre.length),Arrays.copyOfRange(in, i+1, in.length));
           }
        }
        return root;
    }
}

Discuss

采用递归的方式，把第一个步骤考虑清楚即可，注意for循环只采用一次。

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