Question

我有两个数组：

a = [a1，a2，a3，a4，..，an]按升序排序;

b = [b1，b2，b3，...，bm]按升序排序;

我想知道数组a中元素b的元素的位置。

有没有一种快速的方法可以做到而不是一个一个地找到它？

Answer 1

如上所述对数组进行排序，您可以执行二进制搜索。

import java.util.Arrays;

class ArrayTest {
    public static void main(String[] args) {
        int a[] = { 1, 2, 5, 7 };
        int b[] = { -2, 2, 3, 4, 5, 6 };
        for (int i = 0; i < b.length; i++) {
            int position = Arrays.binarySearch(a, b[i]);
            if (position >= 0)
                System.out.println("Element " + b[i] + " is at " + position
                        + " in array A");
            else {
                // System.out.println("Element does not exist in array A");
                int lower = -1;
                int upper = -1;
                for (int j = 0; j < a.length; j++) {
                    if (b[i] > a[j]) {
                        lower = j;
                    } else {
                        upper = j;
                    }
                    if (upper != -1)
                        break;
                }
                if (upper <= 0)
                    System.out.println("Element " + b[i] + " is at before 0");
                else
                    System.out.println("Element " + b[i] + " is Between "
                            + lower + " and " + upper);
            }
        }
    }
}

我使用内置库进行二进制搜索。希望能帮助到你！我已经更新了代码。

Answer 2

O（M）+ O（n）的

    //assuming both a and b are sorted in ascending order
    int startA = 0, startB = 0;
    int[] a = {2,4,6,8,9};
    int[] b = {1,4,9};
    int[] result = new int[b.length];
    //result[i] suggest position of ith element of array b in array a. you can initialize all elements of result to -1 indicating not present in a
    int endB = b.length, endA = a.length;
    while(startB<endB && startA<endA){
        if(b[startB]<a[startA]){
            startB++;
        }
        else if(b[startB]>a[startA]){
            startA++;
        }
        else{
            result[startB]=startA;
            startB++;
        }

    }

Java：从另一个数组中查找数组中元素的位置

2 个答案: