Java面试题系列--经典算法题(持续更新)

package pers.yijin.demo;

import java.util.HashMap;
import java.util.Map;

/**
* 统计字符串中，字符出现的次数
*
* @author Administrator
*
*/
public class CountWords {
/**
* 利用map的键不能重复的特性来实现
*
* @param str
*/
public static void count(String str) {
Map<Character, Integer> map = new HashMap<Character, Integer>();
for (int i = 0; i < str.length(); i++) {
char key = str.charAt(i);
if (!map.containsKey(key)) {
map.put(key, 1);
} else {
int value = map.get(key);
map.put(key, value + 1);
}
}
for (Map.Entry<Character, Integer> m : map.entrySet()) {
System.out.println(m.getKey() + "的次数为：" + m.getValue());
}
}

public static void main(String[] args) {
CountWords.count("askadklfjalkjflfaflkdnmcnvkjfklasdj");
}

}

package pers.yijin.demo;

import java.math.BigInteger;

/**
* 递归调用
*
* @author Administrator
*
*/
public class Fibonacci {
/**
* 斐波那契数列：求1,1,2,3,5,8,13....的第100位的值
*
* @param n
* @return
*/
public static int fibonacci(int n) {
if (n <= 2) {
return 1;
} else {
return fibonacci(n - 1) + fibonacci(n - 2);
}
}

/**
* 求N的阶乘
*
* @param n
* @return
*/
public static int factorial(int n) {
if (n <= 1) {
return 1;
}
return n * factorial(n - 1);
}

/**
* 求最大公约数
*
* @param p
* @param q
* @return
*/
public static int gcb(int p, int q) {
if (q == 0) {
return p;
}
int r = p % q;
return gcb(q, r);
}

public static void main(String[] args) {
System.out.println(BigInteger.valueOf(fibonacci(10)));
System.out.println(factorial(10));
System.out.println(gcb(5, 10));

}

}

package pers.yijin.demo;

import java.util.Arrays;

/**
* 荷兰国旗问题
*  将0、1、2三个数排列，0在左边，1在中间，2在右边
*
* @author Administrator
*
*/
public class FlagOfNetherland {

public static void sort(int[] arr, int low, int high) {
int begin = low;//开始索引
int end = high;//结尾索引
int current = low;//遍历索引
while (current <= end) {
if (arr[current] < 1) {
//当前值==0则与开始索引交换，遍历索引和开始索引+1，则开始索引左边的数据就固定好了(除了开始时，begin永远指向1)
swap(arr, begin, current);
begin++;
current++;
} else if (arr[current] > 1) {
swap(arr, current, end);
end--;
} else {
current++;
}
}
}

// 交换算法
private static void swap(int[] arr, int current, int next) {
int temp = arr[current];
arr[current] = arr[next];
arr[next] = temp;
}

public static void main(String[] args) {

int[] arr = { 0, 1, 2, 1, 2, 0, 2, 1, 0, 2, 0, 1, 2, 1, 2, 0, 2, 1, 0, 2, 0, 1, 2, 1, 2, 0, 2, 1, 0, 2, 0, 1, 2,
1, 2, 0, 2, 1, 0, 2, 0, 1, 2, 1, 2, 0, 2, 1, 0, 2 };
sort(arr, 0, arr.length - 1);
System.out.println(Arrays.toString(arr));
}

}

package pers.yijin.demo;
/**
* 判断一个数是否为素数
* @author Administrator
*
*/
public class IsPrime {

/**
* Math.sqrt()求一个数的非负平方根
* 以平方为界
* @param num
* @return
*/
public static boolean isPrime(int num) {
// 素数最小为2
if (num < 2) {
return false;
}
for (int i = 2; i <= Math.sqrt(num); i++) {
if (num % i == 0) {
return false;
}
}
return true;
}

public static void main(String[] args) {
for (int i = 101; i < 201; i++) {
if(isPrime(i)){
System.out.println(i);
}
}
}

}

package pers.yijin.demo;

/**
* 约瑟环问题 实现30个人轮流报数，9出局，循环，剩下15个人。
*
* @author Administrator
*
*/
public class JosephRing {

public static void main(String[] args) {
// 创建一个约瑟环
boolean[] people = new boolean[30];
for (int i = 0; i < people.length; i++) {
people[i] = true;
}
//用于循环的人数
int personSum = people.length;
//计数的个数
int countNum = 0;
//索引数
int index = 0;
while (personSum > 15) {
//如果当前index已经出局，则让下一个出局
if(people[index]){
countNum++;
//如果数到9，则9出局
if (countNum == 9) {
countNum = 0;
people[index] = false;//index=8时people[index]是第九个人
personSum--;
}
}
index++;
//循环结束，重新开始
if (index == people.length) {
index = 0;
}
}

for (int i = 0; i < people.length; i++) {
System.out.println("第" + i + "个的状态是：" + people[i]);
}
}

}

package pers.yijin.demo;
/**
* 给定一串数字，把它以中文的形式输出
* @author Administrator
*
*/
public class NumberParseString {

public static void convert(int number) {
//数字对应的汉字
String[] num = {"零","一","二","三","四","五","六","七","八","九"};
//单位
String[] unit = {"","十","百","千","万","十万","百万","千万","亿","十","百","千","万亿"};
//将输入数字转换为字符串
String strNum = String.valueOf(number);
//判断0出现了几次
boolean zeroFlag = true;
//结果 字符串
String str = "";
int length = strNum.length();
for (int i = 0; i < length; i++) {
//两个字符做运算，自动转换为数字
int c = strNum.charAt(i)-'0';
if(c != 0) {
str += num[c]+unit[length-i-1];
zeroFlag = true;
}
if(c==0 && zeroFlag && i!=length-1){
str += num[c];
zeroFlag = false;
}
}
System.out.println(str);
}
public static void main(String[] args) {
convert(120);
}

}

package pers.yijin.demo;

/**
* 不使用parseInt把字符串转换为数字
*
* @author Administrator
*
*/
public class ParseInt {
/**
* 利用两个char型运算自动转换为int的特性
*
* @param str
* @return
*/
public static int parseInt(String str) {
int res = 0;
for (int i = 0; i < str.length(); i++) {
char c = str.charAt(i);
if ((c >= '0') && (c <= '9')) {
res = res * 10 + (c - '0');
}
}
return res;
}

public static void main(String[] args) {
System.out.println(parseInt("123456789"));
}

}

package pers.yijin.demo;

/**
* 数字反转
*
* @author Administrator
*
*/
public class Reverse {

public static int reverse(int num) {
StringBuilder builder = new StringBuilder();
String str = builder.append(num).reverse().toString();
return Integer.parseInt(str);

}

public static void main(String[] args) {
System.out.println(reverse(852147963));
}
}

package pers.yijin.demo;
/**
* 求100~999之间的水仙花数
* 水仙花数：例如153是1的3次方+5的3次方+3的3次方的和。
* @author Administrator
*
*/
public class Shuixianhuashu {
/**
* 分解出个位，十位，和百位
* @param num
* @return
*/
public static boolean isShuiXianHuaShu(int num) {
int i = num / 100;
int j = (num % 100) / 10;
int k = num % 10;
if (num == i * i * i + j * j * j + k * k * k) {
return true;
}
return false;
}

public static void main(String[] args) {
for (int i = 100; i < 1000; i++) {
if(isShuiXianHuaShu(i)){
System.out.println(i);
}
}
}

}