Find median in four (individually) sorted arrays with O(1) space

问题

I have an assignment to find a median in 4 individually sorted arrays.

A median is defined as the element that is in the middle of the array (at index floor(N/2))

Requirements:

time complexity: linear to the size of the combined array
space complexity: O(1)

I know how to find a median in 2 sorted arrays with O(1) space and O(logn) time, but I cant find a good solution for 4 arrays that meets the requirement of O(1) space.

I have tried to adjust the algorithm for 3 arrays but it didn't work very well for me.

example for my assignment:

A = {1 5 10 15 20}
B = {2 3 4 6 7}
C = {25 30 35 40 45}
D = {8 9 90 100 145}

median(A,B,C,D) = 10

Thanks in advance

回答1:

Think of a single unsorted array

Consider thinking about the 4 arrays as a single unsorted array, broken into 4 parts. If you can modify the arrays, you can sort all 4 arrays as if 1 by swapping values between them (some optimizations can be made since you know the 4 arrays are sorted). Once you've sorted the arrays up to n/2 (where n is the aggregate length of the 4 arrays), just return the middle value of all 4.

Some Code

The implementation below begins to make multiple arrays function like a single one. I've implemented get, set, and length methods, the basis for any array. All that needs to happen now is for the class' data to be sorted (possibly up to n/2) using get(int), set(int,int), and length(), and a method which returns the median value median().

Perhaps there is a way to get the median value of an array in a single pass, I cannot think of it however. A quick sort and lookup will perform in O(nLogn) time complexity, only a single pass will reduce this to O(n) (linear to the size of the array).

There is also room for further optimization by sorting only up to n/2 within the median method, also when caching (i,j) pairs for each element when doing so.

int median( int[] a1, int[] a2, int[] a3, int[] a4 ) {
    MultiIntArray array = new MultiIntArray( a1, a2, a3, a4 );
    array.sort();
    return array.get( array.length() / 2 );
}

public class MultiIntArray {

    private int[][] data;

    public MultiIntArray( int[]... data ) {
        this.data = data;
    }

    public void sort() {
        // FOR YOU TO IMPLEMENT
    }

    public int length() {
        int length = 0;
        for ( int[] array : data ) {
            length += array.length;
        }
        return length;
    }

    public int get( int index ) {
        int i = 0;
        while ( index >= data[i].length ) {
            index -= data[i].length;
            i += 1;
        }
        return data[i][index];
    }

    public void set( int index, int value ) {
        int i = 0;
        while ( index >= data[i].length ) {
            index -= data[i].length;
            i += 1;
        }
        data[i][index] = value;
    }

}

回答2:

With the restriction "time complexity is the size of the combined arrays" it is trivial, you just pick the smallest of the first elements of four arrays n/2 times. No clever algorithm needed.

I'm sure you can do it significantly faster.

来源：https://stackoverflow.com/questions/55208228/find-median-in-four-individually-sorted-arrays-with-o1-space

标签

arrays

algorithm

median