package com.jnwork;
public class SquenceTest {
/**
* 求和:1 + 1/2! + 1/3! + 1/4! + 1/5! + ... + 1/50!
* @param args
*/
public static void main(String[] args) {
System.out.println("递归求和:" + SquenceTest.getSquenceSum(50));
System.out.println("迭代求和:" + getSequenceSumByInterator(50));
}
/**
* 递归求和
* @param n
* @return
*/
public static double getSquenceSum(int n) {
long startTime = System.currentTimeMillis();
if(n == 1) {
return 1d;
}
double sum = 1d;
double a = 1d;
for(int i = 2; i <= n; i++) {
long num = getSquenceValueByRecurvise(i);
sum += a / num;
}
System.out.println("递归求和耗时:" + (System.currentTimeMillis() - startTime) + "ms");//0ms
return sum;
}
/**
* 递归方式
* @param n
* @return
*/
public static long getSquenceValueByRecurvise(int n) {
if(n == 1) {
return 1;
}
long squenceValue = n * getSquenceValueByRecurvise(n-1);
return squenceValue;
}
/**
* 迭代求和
*/
public static double getSequenceSumByInterator(int n) {
long startTime = System.currentTimeMillis();
if(n == 1) {
return 1d;
}
double sum = 1d;
double a = 1d;
long beforeSeqValue = 1;
long nextSeqValue = 0;
for(int i = 2; i <= n; i++ ) {
//迭代关系(an = n * a(n-1) an-1 = an;)
nextSeqValue = i * beforeSeqValue;//相当于数学中的递推公式
beforeSeqValue = nextSeqValue;
sum += a / nextSeqValue;
}
/*while(n > 1) {
nextSeqValue = n * beforeSeqValue;
beforeSeqValue = nextSeqValue;
n--;
}*/
System.out.println("迭代求和耗时:" + (System.currentTimeMillis() - startTime) + "ms");//0ms
return sum;
}
}