java - 使用蛮力的其他数字乘积 InterviewCake 答案

Question

我正在尝试使用蛮力来回答这个问题，这样我就可以理解发生了什么:

https://www.interviewcake.com/question/java/product-of-other-numbers

但问题是我不明白为什么我的逻辑不起作用。到目前为止，这是我尝试过的:

public class EveryInteger {

    public static void main(String[] args) {

        int a1[] = {1, 7, 3, 4};
         int a2[] = new int[4];
          for(int i =0; i<a1.length; i++){
            int index = a1[i];
            int sum = 0;

            for(int j =0; j<a1.length; j++){
                if(a1[i]==a1[j]){
                    continue;
                }

                sum = index * a1[j];
                a2[i] = sum;
                System.out.println(a2[i]);
            }
          }
    }

谁能告诉我你是如何使用两个 for 循环解决这个问题的？

Answer 1

这里有几个问题:

// defining an array like this is confusing because the type is not immediately clear, rather use int[] a1 = ...
int a1[] = {1, 7, 3, 4}; 
// a2 should always have the same length as a1, so use a1.length
int a2[] = new int[4];
for(int i =0; i<a1.length; i++){
    // index is a little confusing, since it's not the index but the value at i
    int index = a1[i];
    // sum is also confusing, since you're working with multiplication here
    // Additionally with multiplication involved, initialize that to 1
    int sum = 0;

    for(int j =0; j<a1.length; j++){
        // comparing only j and i would be sufficient here without accessing the array twice
        if(a1[i]==a1[j]){
            continue;
        }

        // Instead of accumulating the product you reassign it every time, effectively deleting the previous value.
        sum = index * a1[j];
        a2[i] = sum;
        System.out.println(a2[i]);
    }
}

解决方案可能如下所示:

int[] input = {1,7,3,4};
int[] output = new int[input.length];

for(int i = 0; i < input.length; i++) {
  // Accumulates the multiplications.
  int acc = 1;
  for(int j = 0; j < input.length; j++) {
    // check, if not at the same index.
    if(j != i) {
      // only then multiply the current number with the accumulator.
      acc *= input[j];
    }
  }
  // finally write the accumulated product to the output array.
  output[i] = acc;
}

System.out.println(Arrays.toString(output));

结果如愿:

[84, 12, 28, 21]