保研机试准备之常用机试代码

Java快速IO

 

static StreamTokenizer sc = new StreamTokenizer(new BufferedReader(
            new InputStreamReader(System.in)));
    static PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
        int n = nextInt();
        int m = nextInt();
        out.flush();
        out.close();
    public static int nextInt() throws IOException {
        sc.nextToken();
        return (int) sc.nval;
    }

 

并查集

 

/**
 * Union Find Set
 * @author andong
 *
 */
import java.util.Arrays;
public class UFS {
    
    final int MAX = 20;
    int[] par;
    int[] rank;
    
    public UFS(){
        
        par = new int[MAX];
        rank = new int[MAX];
        for(int i=0;i<MAX;i++){            
            par[i] = i;
        }
        Arrays.fill(rank, 1);
    }
    
    public int find(int x){
        
        int px = x;
        while(px!=par[px]){
            px = par[px];
        }
        int t;
        while(x!=px){
            t = par[x];
            par[x] = px;
            x = t;
        }
        return px;
    }
    
    public void union(int x,int y){
        
        int px = find(x);
        int py = find(y);
        if(px != py){
            if(rank[px]>=rank[py]){
                par[py] = px;
                rank[px]+=rank[py];
            }
            else{
                par[px] = py;
                rank[py]+=rank[px];
            }                
        }
    }
    
    public int count(){
        int cnt = 0;
        for(int i=0;i<MAX;i++){
            if(par[i]==i)
                cnt++;
        }
        return cnt;
    }
}

 

拓扑排序

 

public class Topo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[][] g = new int[n+1][n+1];
        int m = sc.nextInt();
        for(int i=0;i<m;i++){
            int a = sc.nextInt();
            int b = sc.nextInt();
            g[a][b] = 1;
        }
        int[] cnt = new int[n+1];
        boolean[] in = new boolean[n+1];
        Arrays.fill(in, false);
        Arrays.fill(cnt, 0);
        for(int i=1;i<=n;i++){
            for(int j=1;j<=n;j++){
                if(g[i][j]==1)
                    cnt[j]++;
            }
        }
        Queue<Integer> q = new LinkedList<Integer>();
        for(int i=1;i<=n;i++){
            if(cnt[i]==0){
                q.add(i);
                in[i] = true;
            }
        }
        while(!q.isEmpty()){
            int p = q.remove();
            for(int i=1;i<=n;i++){
                if(g[p][i]==1){
                    cnt[i]--;
                    if(cnt[i]==0&&!in[i]){
                        q.add(i);
                        in[i] = true;
                    }    
                }
            }
            System.out.print(p);            
        }
    }
}

 

最小生成树的Kruskal算法

 

import java.util.ArrayList;
import java.util.Comparator;
import java.util.PriorityQueue;
/**
 * Kruskal算法练习
 * @author andong
 *
 */
public class Kruskal {
    public static final int INF = Integer.MAX_VALUE;
    public static int nV;
    public static int[][] graph;
    public static UFS ufs;
    public static PriorityQueue<Edge> priq;
    public static ArrayList<Edge> list;
    public static void init(int[][] g){
        nV = g.length;
        graph = g;
        ufs = new UFS(nV);
        Comparator<Edge> com = new Comparator<Kruskal.Edge>() {
            @Override
            public int compare(Edge o1, Edge o2) {
                return o1.w<o2.w?-1:1;
            }
        };
        priq = new PriorityQueue<Edge>(nV,com);
        list = new ArrayList<Edge>();
        //i->j的图
        for(int i=0;i<nV;i++){
            for(int j=0;j<nV;j++){
                if(graph[i][j]>0&&graph[i][j]<INF){
                    priq.add(new Edge(i, j,graph[i][j]));
                }
            }
        }
        work();
    }
    public static void work(){
        int cnt = 0;
        while(!priq.isEmpty()){
            Edge edge = priq.remove();
            if(ufs.find(edge.s)!=ufs.find(edge.e)){
                ufs.union(edge.s, edge.e);
                list.add(edge);
                cnt++;
            }
            if(cnt==nV-1){
                output();
                break;
            }
        }
    }
    
    public static void output(){
        for(int i=0;i<list.size();i++){
            System.out.println(list.get(i).w);
        }
    }
    
    static class Edge {
        int s,e,w;
        public Edge(int s,int e,int w){
            this.s = s;
            this.e = e;
            this.w = w;
        }
    }
    
    public static void main(String[] args) {
        int[][] a = {{0,1,2,4},{0,0,2,5},{0,0,0,3},{0,0,0,0}};
        Kruskal.init(a);
        
    }
}

 

最短路径的Dijkstra算法

 

import java.util.ArrayList;
import java.util.Comparator;
import java.util.PriorityQueue;
/**
 * Dijkstra算法练习
 * @author andong
 *
 */
public class Dijkstra {
    public static int nV;
    public static int src;
    public static int[][] graph;
    public static int[] dist;
    public static ArrayList<Integer> list;
    public static PriorityQueue<point> priq;
    
    public static void init(int[][] g,int s){
        nV = g.length;
        src = s;
        graph = g;
        dist = new int[nV];
        list = new ArrayList<Integer>();
        Comparator<point> com = new Comparator<point>() {
            public int compare(point o1, point o2) {
                return o1.dist<o2.dist?-1:1;
            }
        };
        priq = new PriorityQueue<point>(nV,com);
        
        for(int i=0;i<nV;i++){
            dist[i] = graph[src][i];
            if(i!=src)    priq.add(new point(i, dist[i]));
        }
        
    }
    
    public static void work(){
        while(!priq.isEmpty()){
            point p = priq.remove();
            for(int i=0;i<nV;i++){
                int tmp = dist[p.pos]+graph[p.pos][i];
                if(tmp<dist[i]){
                    dist[i] = tmp;
                    priq.add(new point(i, dist[i]));
                }
            }
            list.add(p.pos);
        }
        
    }
    
    public static void output(int dest){
        System.out.println(dist[dest]);
        for(int i=0;i<list.size();i++){
            System.out.print(list.get(i));
            if(list.get(i)==dest){
                System.out.println();
                break;
            }
        }
    }
    
    static class point{
        int pos,dist;
        public point(int pos,int dist){
            this.pos = pos;
            this.dist = dist;
        }
    }
    
    public static void main(String[] args) {
        int[][] a = {{0,1,2,4},{0,0,2,5},{0,0,0,3},{0,0,0,0}};
        Dijkstra.init(a, 0);
        for(int i=0;i<4;i++){
            Dijkstra.output(i);
        }
    }
}

 

最短路径的Floyd算法

 

import java.util.Arrays;
/**
 * Floyd算法练习
 * @author andong
 *
 */
public class Floyd {
    public static int nV;
    public static int[][] dist;
    public static int[][] inter;
    public static void init(int[][] g){
        nV = g.length;
        dist = new int[nV][nV];
        for(int i=0;i<nV;i++)
            for(int j=0;j<nV;j++){
                if(g[i][j]<Integer.MAX_VALUE)
                    dist[i][j] = g[i][j];
                else
                    dist[i][j] = -1;
            }
        inter = new int[nV][nV];
        for(int i=0;i<nV;i++){
            for(int j=0;j<nV;j++){
                inter[i][j] = j;
            }
        }
        work();
    }
    
    public static void work(){
        for(int k=0;k<nV;k++){
            for(int i=0;i<nV;i++){
                for(int j=0;j<nV;j++){
                    if(dist[i][k]>0&&dist[k][j]>0&&dist[i][j]>dist[i][k]+dist[k][j]){
                        dist[i][j] = dist[i][k]+dist[k][j];
                        inter[i][j] = k;
                    }
                }
            }
        }
    }
    
    public static void outputDist(int src,int dest){
        System.out.println(dist[src][dest]);
    }
    
    public static void outputPath(int src,int dest){
        if(src==dest){
            System.out.print(src);
            return;
        }
        System.out.print(src);
        outputPath(inter[src][dest], dest);
    }
    
    public static void main(String[] args) {
        int INF = Integer.MAX_VALUE;
        int[][] a = {{0,1,2,4},{INF,0,2,7},{INF,INF,0,3},{INF,INF,INF,0}};
        Floyd.init(a);
        for(int i=0;i<4;i++)
            outputDist(1, i);
        outputPath(1, 3);
    }
}

 

由前序序列和中序序列构建树

 

1)序列用字符串表示

    

　　public static void build(String a,String b){
        int len = a.length();
        if(len==1){
            System.out.println(a);
        }
        else if(len>1){
            char c = a.charAt(0);
            String[] temp = b.split(c+"");
            int len0 = temp[0].length();
            int len1 = temp[1].length();
            build(a.substring(1, 1+len0),temp[0]);
            build(a.substring(len-len1),temp[1]);
            System.out.print(c);
        }

    }

 

2)序列用整数数组表示

    

　　public static void build(int pos, int[] pre, int[] in) {
        if (pre.length <= 0)
            return;
        tree[pos] = pre[0];
        int idx = 0;
        for (idx = 0; idx < pre.length; idx++) {
            if (in[idx] == pre[0])
                break;
        }
        if (idx > 0 && pos * 2 < tree.length) {
            int[] leftpre = new int[idx];
            int[] leftin = new int[idx];
            System.arraycopy(pre, 1, leftpre, 0, idx);
            System.arraycopy(in, 0, leftin, 0, idx);
            build(pos * 2, leftpre, leftin);
        }
        if (idx < pre.length - 1 && pos * 2 + 1 < tree.length - 1) {
            int[] rightpre = new int[pre.length - idx - 1];
            int[] rightin = new int[in.length - idx - 1];
            System.arraycopy(pre, 1 + idx, rightpre, 0, pre.length - 1 - idx);
            System.arraycopy(in, 1 + idx, rightin, 0, in.length - 1 - idx);
            build(pos * 2 + 1, rightpre, rightin);
        }
    }