求和：1 + 1/2! + 1/3! + 1/4! + 1/5! + ... + 1/50!

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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;      }  }
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