POJ 刷题系列：2255. Tree Recovery

POJ 刷题系列：2255. Tree Recovery

传送门：POJ 2255. Tree Recovery

题意：

给出先序和中序，求后序。

思路： 依旧很水，给定先序易知根结点，从中序中找到根结点就能确定左孩子们和右孩子们，递归建树。最后后序遍历一遍即可。

代码如下：

代码语言：javascript复制

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Map;
import java.util.StringTokenizer;

public class Main{

    String INPUT = "./data/judge/201712/P2255.txt";

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    class TreeNode{
        char val;
        TreeNode left;
        TreeNode right;

        TreeNode(char val){
            this.val = val;
        }
    }

    TreeNode build(String preord, String inord) {
        if (inord.isEmpty()) return null;
        if (inord.length() == 1) return new TreeNode(inord.charAt(0));

        char rootVal = preord.charAt(0);
        TreeNode root = new TreeNode(rootVal);
        int idx = inord.indexOf(rootVal);
        String left = inord.substring(0, idx);
        String right = inord.substring(idx   1);
        root.left = build(preord.substring(1, 1   left.length()), left);
        root.right = build(preord.substring(1   left.length()), right);

        return root;
    }

    void postOrder(TreeNode root) {
        if (root == null) return;
        postOrder(root.left);
        postOrder(root.right);
        out.print(root.val);
    }

    void read() {
        while (more()) {
            String preord = ns();
            String inord  = ns();

            TreeNode root = build(preord, inord);
            postOrder(root);
            out.println();
        }
    }

    FastScanner in;
    PrintWriter out;

    void run() throws IOException {
        boolean oj;
        try {
            oj = ! System.getProperty("user.dir").equals("F:\oxygen_workspace\Algorithm");
        } catch (Exception e) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
        }

        InputStream is = oj ? System.in : new FileInputStream(new File(INPUT));
        in = new FastScanner(is);
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        read();
        out.flush();
        if (!oj){
            System.out.println("["   (System.currentTimeMillis() - s)   "ms]");
        }
    }

    public boolean more(){
        return in.hasNext();
    }

    public int ni(){
        return in.nextInt();
    }

    public long nl(){
        return in.nextLong();
    }

    public double nd(){
        return in.nextDouble();
    }

    public String ns(){
        return in.nextString();
    }

    public char nc(){
        return in.nextChar();
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        boolean hasNext;

        public FastScanner(InputStream is) throws IOException {
            br = new BufferedReader(new InputStreamReader(is));
            hasNext = true;
        }

        public String nextToken() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    hasNext = false;
                    return "##";
                }
            }
            return st.nextToken();
        }

        String next = null;
        public boolean hasNext(){
            next = nextToken();
            return hasNext;
        }

        public int nextInt() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Integer.parseInt(more);
        }

        public long nextLong() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Long.parseLong(more);
        }

        public double nextDouble() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Double.parseDouble(more);
        }

        public String nextString(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more;
        }

        public char nextChar(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more.charAt(0);
        }
    }

    static class D{

        public static void pp(int[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row;   i) {
                for (int j = 0; j < col;   j) {
                    sb.append(board[i][j]   (j   1 == col ? "n" : " "));
                }
            }
            System.out.println(sb.toString());
        }

        public static void pp(char[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row;   i) {
                for (int j = 0; j < col;   j) {
                    sb.append(board[i][j]   (j   1 == col ? "n" : " "));
                }
            }
            System.out.println(sb.toString());
        }
    }

    static class ArrayUtils {

        public static void fill(int[][] f, int value) {
            for (int i = 0; i < f.length;   i) {
                Arrays.fill(f[i], value);
            }
        }

        public static void fill(int[][][] f, int value) {
            for (int i = 0; i < f.length;   i) {
                fill(f[i], value);
            }
        }

        public static void fill(int[][][][] f, int value) {
            for (int i = 0; i < f.length;   i) {
                fill(f[i], value);
            }
        }
    }

    static class Num{
        public static <K> void inc(Map<K, Integer> mem, K k) {
            if (!mem.containsKey(k)) mem.put(k, 0);
            mem.put(k, mem.get(k)   1);
        }
    }
}

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