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 {
    /**
     * 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 && vinOrder.length == 0 ) return null;

        List<Integer> preList = new ArrayList<>();
        List<Integer> vinList = new ArrayList<>();

        int preLength = preOrder.length;
        for (int i = 0; i < preLength; i++) {
            preList.add(preOrder[i]);
        }
        int vinLength = vinOrder.length;
        for (int i = 0; i < vinLength; i++) {
            vinList.add(vinOrder[i]);
        }

        TreeNode target = buildTree(preList, vinList, null);

        return target;
    }

    TreeNode buildTree(List<Integer> preList, List<Integer> vinList,
                       TreeNode root) {

        if (preList == null || preList.size() == 0) return null;
        int rootVal = preList.get(0);
        root = new TreeNode(rootVal);
        int rootIndex = vinList.indexOf(rootVal);

        List<Integer> preListLeft = null;
        List<Integer> vinListLeft = null;
        List<Integer> preListRight = null;
        List<Integer> vinListRight = null;
        if (rootIndex > 0) {
            vinListLeft = vinList.subList(0, rootIndex);
            preListLeft = preList.subList(1, rootIndex + 1);
        }

        if (rootIndex < vinList.size() - 1) {
            vinListRight = vinList.subList(rootIndex + 1, vinList.size());
            preListRight = preList.subList(preList.size() - vinListRight.size(),
                                          preList.size());
        }

        TreeNode left = buildTree(preListLeft, vinListLeft, root.left);
        if (left != null) root.left = left;

        TreeNode right = buildTree(preListRight, vinListRight, root.right);
        if (right != null) root.right = right;

        return root;
    }
}