题解 | #重建二叉树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    private Map<Integer, Integer> inMap = new HashMap<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param preOrder int整型一维数组 
     * @param vinOrder int整型一维数组 
     * @return TreeNode类
     */
    public TreeNode reConstructBinaryTree (int[] preOrder, int[] vinOrder) {
        // write code here
        for (int i = 0; i < vinOrder.length; i++) {
            inMap.put(vinOrder[i], i);
        }
        return buildTree(preOrder, vinOrder, 0, preOrder.length - 1, 0, vinOrder.length - 1);
    }

    private TreeNode buildTree(int[] preOrder, int[] vinOrder, int preStart, int preEnd, int inLeft, int inEnd) {
        if (preEnd < preStart) {
            return null;
        }
        int current = preOrder[preStart];
        TreeNode root = new TreeNode(current);
        int currentIndex = inMap.get(current);
        int leftLeng = currentIndex - inLeft;
        root.left = buildTree(preOrder, vinOrder, preStart + 1, preStart + leftLeng, inLeft, currentIndex - 1);
        root.right = buildTree(preOrder, vinOrder, preStart + leftLeng  + 1, preEnd, currentIndex + 1, inEnd);
        return root;
    }
}