POJ 2255 Tree Recovery（根据前中序遍历，重建二叉树并求后序遍历）

题意：给出二叉树的前序遍历和中序遍历，求后序遍历。

NO.1：无需重建二叉树，可直接求出后序遍历结果。

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    private static String last = "";  // 保存后序遍历结果

    private static void getLate(String before, String middle) {
        // 递归终点
        if ("".equals(before) || "".equals(middle) || (middle.indexOf(before.charAt(0)) < 0)) {
            return;
        }
        char temp = before.charAt(0);               // 获取根结点
        int rootMiddleIndex = middle.indexOf(temp); // 获取根结点在中序遍历的位置下标
        // 递归左子树
        getLate(before.substring(1, rootMiddleIndex + 1), middle.substring(0, rootMiddleIndex));
        // 递归右子树
        getLate(before.substring(rootMiddleIndex + 1), middle.substring(rootMiddleIndex + 1));
        // 后序遍历结果：左右中
        last += temp;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        String before;
        String middle;
        while (in.hasNext()) {
            before = in.next();
            middle = in.next();
            last = "";
            getLate(before, middle);
            out.println(last);
        }
        out.flush();
    }
}

NO.2 重建二叉树

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    private static String last;

    // 构建二叉树，并返回根节点
    private static Node constructCore(String before, String middle) {
        if ("".equals(before) || "".equals(middle) || before.length() < 0) {
            return null;
        }
        Node root = new Node();
        char temp = before.charAt(0);
        int rootMiddleIndex = middle.indexOf(temp);
        root.value = temp;
        root.left = constructCore(before.substring(1, rootMiddleIndex + 1), middle.substring(0, rootMiddleIndex));
        root.right = constructCore(before.substring(rootMiddleIndex + 1), middle.substring(rootMiddleIndex + 1));
        return root;
    }

    // 后序遍历
    private static void lastOrder(Node root) {
        if (root == null) {
            return;
        }
        lastOrder(root.left);
        lastOrder(root.right);
        last += root.value;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        String before;
        String middle;
        while (in.hasNext()) {
            before = in.next();
            middle = in.next();
            Node root = constructCore(before, middle);  // 重建之后的二叉树根结点
            last = "";
            lastOrder(root);
            out.println(last);
        }
        out.flush();
    }

}

class Node {
    public Node left;
    public Node right;
    public char value;
}