POJ 2255 Tree Recovery（根据前中序遍历，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;}