Java经典递归算法

斐波那契数列

package com.luna.base;
public class BirthRabbit {
    public static void main(String[] args) {
        int i = 1;
        for (i = 1; i <= 20; i++) {
            System.out.println("兔子第" + i + "个月的总数为:" + f(i));
        }
    }
    public static int f(int x) {
        if (x == 1 || x == 2) {
            return 1;
        } else {
            return f(x - 1) + f(x - 2);
        }
    }
}

从1到100相加

package com.luna.base;
public class Plus {
    public int sum(int i) {
        if (i == 1) {
            return 1;
        }
        return i + sum(i - 1);
    }
    public static void main(String[] args) {
        Plus plus = new Plus();
        System.out.println("计算结果：" + plus.sum(100) + "!");
    }
}

100的阶乘

package com.luna.base;
import java.math.BigInteger;
public class LoopMutiply {
    public BigInteger sum(int i) {
        if (i == 1) {
            return BigInteger.ONE;
        }
        return BigInteger.valueOf(i).multiply(sum(i - 1));
    }
    public static void main(String[] args) {
        LoopMutiply test = new LoopMutiply();
        try {
            System.out.println("计算结果：" + test.sum(50) + "!");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

有序数组a、b合并成一个新的有序数组

package com.luna.base;
public class ArraySort {
    public static void main(String[] args) {
        int[] a = { 1, 3, 4 };
        int[] b = { 2, 3, 5, 6 };
        int[] c = mergeArray(a, b);
        for (int n : c) {
            System.out.print(n + " ");
        }
    }
    // 合并数组
    public static int[] mergeArray(int[] a, int[] b) {
        int result[] = new int[a.length + b.length];
        if (checkSort(a) && checkSort(b)) {
            // 说明ab数组都是有序的数组
            // 定义两个游标
            int i = 0, j = 0, k = 0;
            while (i < a.length && j < b.length) {
                if (a[i] <= b[j]) {
                    result[k++] = a[i++];
                } else {
                    result[k++] = b[j++];
                }
            }
            while (i < a.length) {
                // 说明a数组还有剩余
                result[k++] = a[i++];
            }
            while (j < b.length) {
                result[k++] = b[j++];
            }
        }
        return result;
    }
    // 检查一个数组是否是有序1 2 3
    public static boolean checkSort(int[] a) {
        boolean flag = false;// 默认不是有序的
        for (int i = 0; i < a.length - 1; i++) {
            if (a[i] > a[i + 1]) {
                // 说明不是有序的
                flag = false;
                break;
            } else {
                flag = true;
            }
        }
        return flag;
    }
}

归并排序算法实现

package com.luna.base;
public class MergingSort {    
    public static void sort(int[] data, int left, int right) {    
        if (left < right) {    
            // 首先找出中间的索引    
            int center = (left + right) / 2;    
    
            // 对中间索引左边的数组进行递归    
            sort(data, left, center);    
    
            // 对中间索引右边的数组进行递归    
            sort(data, center + 1, right);    
            // 合并    
            merge(data, left, center, right);    
        }    
    }    
    
    public static void merge(int[] data, int left, int center, int right) {    
        int[] tmpArr = new int[data.length];        
        int mid = center + 1;      
        // third记录中间数组的索引    
        int third = left;    
        int tmp = left;        
        while (left <= center && mid <= right) {    
            // 将两个数组中取出最小的数放入中间数组    
            if (data[left] <= data[mid]) {    
                tmpArr[third++] = data[left++];    
            } else {    
                tmpArr[third++] = data[mid++];    
            }    
        }    
    
        // 剩余部分依次放入中间数组    
        while (mid <= right) {    
            tmpArr[third++] = data[mid++];    
        }       
        while (left <= center) {    
            tmpArr[third++] = data[left++];    
        }                
        while(tmp <= right){    
            data[tmp] = tmpArr[tmp++];    
        }    
    }    
    
    public static void main(String[] args) {    
        int[] a = { 3, 2, 5, 4 };               
        sort(a, 0, a.length - 1);        
        for (int i = 0; i < a.length; i++) {    
            System.out.print(a[i] + " ");    
        }    
    }    
}

九九

public static void main(String[] args) {
for (int i = 1 ; i <= 9 ; i++){
for (int j = 1 ; j <= i ; j++){
System.out.print(i + "*" + j + "=" + i * j + " ");
}
System.out.println();
}
}