题解 | #重建二叉树#

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public TreeNode reConstructBinaryTree(int [] pre,int [] vin) {
        if(pre.length==0||vin.length==0){
            return null;
        }
        return helper( pre, vin,0,pre.length-1,0,vin.length-1);
    }
    public static TreeNode helper
      (int[] pre,int[] vin,int pl,int pr,int vl,int vr){
        if(pl>=pre.length||vl>=vin.length||pl>pr||vl>vr){
            return null;
        }
        int val = pre[pl];
        TreeNode root = new TreeNode(val);
        int count =vl;
        while(vin[count]!=val){
            count++;
        }
        count-=vl;
        root.left =helper(pre,vin,pl+1,pl+count,vl,vl+count-1);
        root.right=helper(pre,vin,pl+count+1,pr,vl+count+1,vr);
        return root;
    }
}