题解 | #重建二叉树#

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) {
            return null;
        }
        if (pre.length == 1) {
            return new TreeNode(pre[0]);
        }
        if (pre.length == 2) {
            TreeNode root = new TreeNode(pre[0]);
            if (pre[0] == vin[0]) {
                root.right = new TreeNode(vin[1]);
            } else {
                root.left = new TreeNode(vin[0]);
            }
            return root;
        }
        int rootVal = pre[0];
        int rootIndex = 0;
        for (int i = 0; i < vin.length; i++) {
            if (vin[i] == rootVal) {
                rootIndex = i;
                break;
            }
        }
        TreeNode root = new TreeNode(rootVal);
        if (rootIndex == 0) {
            int[] rightVin = Arrays.copyOfRange(vin, 1, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : rightVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] rightPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        } else if (rootIndex == 1) {
            root.left = new TreeNode(vin[0]);
            int[] rightVin = Arrays.copyOfRange(vin, 2, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : rightVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] rightPre = Arrays.copyOfRange(pre, 2, endIndex + 1);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        } else if (rootIndex == vin.length - 1) {
            int[] leftVin = Arrays.copyOfRange(vin, 0, vin.length - 1);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.left = reConstructBinaryTree(leftPre, leftVin);
        } else if (rootIndex == vin.length - 2) {
            root.right = new TreeNode(vin[vin.length - 1]);
            int[] leftVin = Arrays.copyOfRange(vin, 0, vin.length - 2);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.left = reConstructBinaryTree(leftPre, leftVin);
        } else {
            int[] leftVin = Arrays.copyOfRange(vin, 0, rootIndex);
            int[] rightVin = Arrays.copyOfRange(vin, rootIndex + 1, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            int[] rightPre = Arrays.copyOfRange(pre, endIndex + 1, pre.length);
            root.left = reConstructBinaryTree(leftPre, leftVin);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        }
        return root;
    }
}