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> map = 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
        if (preOrder.length == 0) {
            return null;
        }
        for (int i = 0; i < vinOrder.length; ++i) {
            map.put(vinOrder[i], i);
        }

        return reBuild(preOrder, 0, preOrder.length - 1, vinOrder, 0, vinOrder.length - 1);

    }

    private TreeNode reBuild(int[] preOrder, int pl, int pr, int[] vinOrder, int vl, int vr) {
        TreeNode node = new TreeNode(preOrder[pl]);
        int vinIndex = map.get(preOrder[pl]);
        if (pl + 1 <= pl + vinIndex - vl) {
            node.left = reBuild(preOrder, pl + 1, pl + vinIndex - vl, vinOrder, vl, vinIndex - 1);
        }
        if (pl + vinIndex - vl + 1 <= pr) {
            node.right = reBuild(preOrder, pl + vinIndex - vl + 1, pr, vinOrder, vinIndex + 1, vr);
        }
        return node;
    }
}