java 常用算法和一些题目
选择排序，复杂度O（n²）
package com.example.demo;
import org.junit.Test;
/**
* 选择排序
* @author zhzh.yin
*
*/
public class HTest {
@Test
public void testMethod(){
Integer [] numList = {1,2,2,2,1,4,5,2,5,3,9,6};
for(int i=0;i<numList.length-1;i++){
int max = numList[i];
for(int j = i+1;j<numList.length;j++){
if(numList[i]<numList[j]){
max = numList[j];
numList[j]=numList[i];
numList[i]=max;
}
}
}
for(int num:numList){
System.out.print(numList[num]+" ");
}
}
}
冒泡排序 复杂度 O(n2)
package com.example.demo;
import org.junit.Test;
/**
* 冒泡排序
* @author zhzh.yin
*
*/
public class FTest {
@Test
public void testMethod(){
Integer[]numList = {1,2,1,1,1,8,3,5,1};
if(numList.length==1){
System.out.println("-1");
}else if(numList.length==2){
String string ="";
System.out.println(
numList[1]>numList[2]?
numList[1]+""+numList[0]:numList.toString());;
}
int temp = 0;
for(int i =0;i<numList.length-1;i++){
for(int j=i+1;j<numList.length;j++){
if(numList[i]<numList[j]){
temp=numList[i];
numList[i]=numList[j];
numList[j]=temp;
}
}
}
for(int num :numList){
System.out.print(num+" ");
}
}
}
快速排序O（nlogn）
@Test
public void speedSortTest(){
Integer []numList={2,3,7,9,5,10,1,4};
speedSort(numList,0,numList.length-1);
for(int time:numList){
System.out.print(time+" ");
}
}
public void speedSort(Integer []numList,int lower,int higher){
if(lower>=higher){
return;
}
int temp=0;
int icon = numList[lower];
int indexofLeft = lower;
int indexofRight = higher;
while(indexofLeft<indexofRight){
while(icon<=numList[indexofRight]&&indexofLeft<indexofRight){
indexofRight--;
}
while(icon>=numList[indexofLeft]&&indexofLeft<indexofRight){
indexofLeft++;
}
if(indexofLeft<indexofRight){
temp = numList[indexofRight];
numList[indexofRight]=numList[indexofLeft];
numList[indexofLeft]=temp;
}
for(int num:numList){
System.out.print(num+" ");
}
System.out.println();
}
temp=numList[indexofLeft];
numList[indexofLeft]=numList[lower];
numList[lower]=temp;
speedSort(numList,lower,indexofLeft-1);
speedSort(numList,indexofRight+1,higher);
}
二分法查询-非迭代O（logn）
http://hedengcheng.com/?p=595
import org.junit.Test; /** * 找到数字N，适用于有多个重复数字，使用部分math原生方法向上取整/向下取整 * middle=min+(max-min)/2 算法更加准确
* 返回-1为没有找到 * @author zhzh.yin * */ public class CTest { @Test public void testMethod(){ Integer[] num ={1,2,3,3,3,4,5,5,5,5,5,6,6,6,7,10}; System.out.println("the num is at "+selectPosition(num,8)); } public int selectPosition(Integer [] numList,int num){ int min=0; int max=numList.length-1; int middle = -1; while(min<=max){ middle = min+(max-min)/2; System.out.println("min--"+min+",max--"+max+",middle--"+middle); if((min==middle||max==middle)&&numList[middle]!=num){ middle = -1; break; } if(numList[middle]<num){ min=(int) Math.floor(middle); }else if(numList[middle]>num){ max=(int) Math.ceil(middle); }else{ break; } } return middle; } }
迭代+复杂化二分法
package com.example.demo;
import org.junit.Test;
/**
* 找出数组中最后一个6(或者8)的位置,复杂度O(nlogn)，用迭代实现
*
* @author zhzh.yin flag==true 获取最右的坐标
*
*/
public class CTest {
@Test
public void testMethod() {
Integer[] num = { 1, 2, 3, 3, 3, 4, 5, 5, 5, 5, 5, 6, 6, 6, 7, 10 };
System.out.println("hello,position is " + selectPosition(num, 10, 0, num.length - 1, false));
}
public int selectPosition(Integer[] numList, int num, int min, int max, Boolean flag) {
int middle = min + (max - min) / 2;
System.out.println("min-" + min + ",max-" + max + ",middle-" + middle);
if (min < max - 1) {
if (numList[middle] < num) {
return selectPosition(numList, num, (int) Math.floor(middle), max, flag);
} else if (numList[middle] > num) {
return selectPosition(numList, num, min, (int) Math.ceil(middle), flag);
} else if (numList[middle] == num && (min != max && min != max - 1)) {
if (flag) {
return selectPosition(numList, num, (int) Math.ceil(middle), max, flag);
} else {
return selectPosition(numList, num, min, (int) Math.floor(middle), flag);
}
}
} else{
if (numList[min] != num && numList[max] != num) {
middle = -1;
return middle;
} else if (numList[min] == numList[max]) {
return flag.equals(true) ? max : min;
} else if (numList[min] == num) {
return min;
} else {
return max;
}
}
return middle;
}
}
顺时针打印矩阵
递归
import org.junit.Test;
/**
* 顺时针打印矩阵
* @author user
*
*/
public class ATest {
//顺时针打印一个矩阵
@Test
public void test(){
int[][] num = new int[100][100];
int n = 6;
int count =1;
for(int i=0;i<n;i++){
for(int j =0;j<n;j++){
num[i][j]=count++;
}
}
output(num,0,n-1);
}
/**
*
* @param num
* @param start
* @param end
*/
public void output(int[][] num,int start,int end){
if(start>end || end<=0)return;
for(int i=start;i<=end;i++){
System.out.println(num[start][i]);
}
for(int i=start+1;i<=end;i++){
System.out.println(num[i][end]);
}
for(int i=end-1;i>=start;i--){
System.out.println(num[end][i]);
}
for(int i=end-1;i>start;i--){
System.out.println(num[i][start]);
}
output(num,start+1,end-1);
}
}
找出出现次数最多的
先recordNum 再从hashMap里面找出value最大的
package com.example.demo;
import java.util.HashMap;
import java.util.Map;
import org.junit.Test;
/**
* 找出出现次数最多的数值
* @author zhzh.yin
*
*/
public class BTest {
@Test
public void TestMethod(){
Integer []numList = {1,1,1,-1,2,3,44,4,5,6,9,9,9,9,9,8,7,6,5};
// chooseMax(recordNum(numList));
System.out.println(chooseMax(recordNum(numList),numList[0]));
}
public HashMap <Integer,Integer> recordNum (Integer [] numList){
HashMap<Integer, Integer> map=new HashMap<>();
for(int num:numList){
if(map==null){
map.put(num, 1);
continue;
}
if(map.containsKey(num)){
map.put(num, map.get(num)+1);
}else{
map.put(num, 1);
}
}
return map;
}
public String chooseMax(HashMap<Integer, Integer>map,int num){
String string = "";
int key = num;
int time = map.get(key);
for(int keyActually :map.keySet()){
if(map.get(keyActually)>time){
time=map.get(keyActually);
key = keyActually;
}
}
string= "出现次数最多的数是"+key+" "+time+" "+map.toString();
return string;
}
}
把AB文件字符合并重排，输出到C
package com.example.demo;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.junit.Test;
/**
* 把AB文件中的字符重排，输入到C
* @author zhzh.yin
*
*/
public class ETest {
@Test
public void TestMethod() throws IOException{
FileReader filea = new FileReader("C:\\Users\\user\\Downloads\\a.txt");
BufferedReader ina = new BufferedReader(filea) ;
FileReader fileb = new FileReader("C:\\Users\\user\\Downloads\\b.txt");
BufferedReader inb = new BufferedReader(fileb) ;
String line ;
char[]arr;
List<Character> list = new ArrayList<>();
BufferedWriter writerc= new BufferedWriter(new FileWriter("C:\\Users\\user\\Downloads\\c.txt"));
while((line=ina.readLine())!=null){
arr =line.toCharArray();
for(int i=0;i<arr.length;i++){
list.add(arr[i]);
}
}
ina.close();
while((line=inb.readLine())!=null){
arr =line.toCharArray();
for(int i=0;i<arr.length;i++){
list.add(arr[i]);
}
}
inb.close();
Object[]array = list.toArray();
Arrays.sort(array);
for(Object object:array){
writerc.write((char)object);
}
writerc.close();
FileReader filec = new FileReader("C:\\Users\\user\\Downloads\\c.txt");
BufferedReader read = new BufferedReader(filec);
while((line = read.readLine())!=null){
System.out.println(line);
}
read.close();
}
}
找出出现2次的值
package com.example.demo;
import java.util.HashMap;
import org.junit.Test;
/**
* 找出出现2次的数值
* @author zhzh.yin
*
*/
public class GTest {
@Test
public void testMethod(){
Integer [] numList = {1,2,2,2,3,4,5,5,6,7,7};
HashMap<Integer,Integer> numMap = new HashMap();
for(int num :numList){
if (!numMap.containsKey(num)){
numMap.put(num, 1);
}else {
numMap.put(num, numMap.get(num)+1);
}
}
System.out.println(numMap);
for(HashMap.Entry<Integer, Integer>entry :numMap.entrySet()){
if(entry.getValue()==2){
System.out.print(entry.getKey()+" ");}
}
}
}
