题解 | #重建二叉树#

一定要画图，并使用指针标识

import java.util.*;
/**
 * 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 [] vin) {
        if(pre.length == 0 || vin.length == 0){
            return null;
        }
        
        // 构建根节点
        int rootVal = pre[0];
        TreeNode root = new TreeNode(rootVal);
        
        // 在中序遍历中找到根节点位置
        int rootIndex = 0;
        for(int i = 0; i < vin.length; i++){
            if(vin[i] == rootVal){
                rootIndex = i;
                break;
            }
        }
        
        
        // 构建左子树
        if(rootIndex != 0) {
            int[] leftPre = new int[rootIndex];
            int[] leftVin = new int[rootIndex];
            for(int i = 0; i < rootIndex; i++){
                leftPre[i] = pre[i + 1];
                leftVin[i] = vin[i];
            }
            
            root.left = reConstructBinaryTree(leftPre, leftVin);
        }
        
        
        // 构建右子树
        if(rootIndex != vin.length - 1){
            int[] rightPre = new int[vin.length - rootIndex - 1];
            int[] rightVin = new int[vin.length - rootIndex - 1];
            for(int i = 0; i < vin.length - rootIndex - 1; i++){
                rightPre[i] = pre[rootIndex + i + 1];
                rightVin[i] = vin[rootIndex + i + 1];
            }
            root.right = reConstructBinaryTree(rightPre, rightVin);
        }
        
        
        return root;
    }
    
}