4.重建二叉树

Posted 2021-01-30 yihangzhou

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; }
 * }
 */
import java.util.HashMap;
import java.util.Map;
public class Solution {
    private Map<Integer,Integer> map;
    public TreeNode reConstructBinaryTree(int [] pre, int [] in) {
        map=new HashMap<Integer,Integer>();
        for(int i=0;i<in.length;i++){
            map.put(in[i],i);
        }
        return RecurConstructTree(pre,0,pre.length-1,0);
    }
    public  TreeNode RecurConstructTree(int[] pre,int preL,int preR,int inL){
        if(preL>preR){
            return null;
        }
        TreeNode root=new TreeNode(pre[preL]);
        int index=map.get(root.val);
        int leftSize=index-inL;
        root.left=RecurConstructTree(pre,preL+1,preL+leftSize,inL);
        root.right=RecurConstructTree(pre,preL+leftSize+1,preR,inL+leftSize+1);
        return root;
    }
}