我如何计算排列？ [英] How do I calculate permutation?

问题描述

我对Java中的排列有疑问。

I am having a question about permutation in Java.

假设我在数组中有五个不同的元素[a，b，c，d，e]和我想要从中选择三个元素，顺序确实很重要。我知道在数学中我们可以使用 ₅ P ₃ 来得到答案，但是我们可以使用Java来获取计数和排列集的元素的完整列表< sub> 5 P ₃ ？

Suppose I have five different elements in an array [a, b, c, d, e] and I want to select three elements out of them, the order does matter. I know in math we can use ₅P₃ to get the answer, but can we use Java to get the count and the complete list of elements of the permutation set of ₅P₃?

推荐答案

自1,2,3以来与2,1,3不同，我将建议以下代码片段。请注意，我不能与j或k相同，以避免1,1,1或1,3,3场景。

Since "1, 2, 3" is different from "2, 1, 3", I will suggest the following code snippet. Note that i can't be identical to j or k to avoid "1, 1, 1" or "1, 3, 3" scenarios.

   // array with elements
   char[] items = {'a', 'b', 'c', 'd', 'e'};
   int count = 0;

    // first of the "three"
    for (int i = 0; i < upperBound; i++) {
       // second of the "three"
       for (int j = 0; j < upperBound; j++) {
          // can't be identical to i
          if (j == i) 
             continue;

          // third of the "three"
          for (int k = 0; k < upperBound; k++) {
             // can't be identical to i or j
             if (k == i || k ==j) 
                continue;

             // print some display
             System.out.println(items[i] + ", " + items[j] + ", " + items[k]);

             // increment the total count
             count++;
          }
       }
    }



    System.out.println("Total count is " + count);

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