Codility 17번 NumberOfDiscIntersections

NumberOfDiscIntersections

 

We draw N discs on a plane. The discs are numbered from 0 to N − 1. An array A of N non-negative integers, specifying the radiuses of the discs, is given. The J-th disc is drawn with its center at (J, 0) and radius A[J].

We say that the J-th disc and K-th disc intersect if J ≠ K and the J-th and K-th discs have at least one common point (assuming that the discs contain their borders).

The figure below shows discs drawn for N = 6 and A as follows:

A[0] = 1 A[1] = 5 A[2] = 2 A[3] = 1 A[4] = 4 A[5] = 0

There are eleven (unordered) pairs of discs that intersect, namely:

• discs 1 and 4 intersect, and both intersect with all the other discs;
• disc 2 also intersects with discs 0 and 3.

Write a function:

class Solution { public int solution(int[] A); }

that, given an array A describing N discs as explained above, returns the number of (unordered) pairs of intersecting discs. The function should return −1 if the number of intersecting pairs exceeds 10,000,000.

Given array A shown above, the function should return 11, as explained above.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [0..100,000];
• each element of array A is an integer within the range [0..2,147,483,647].

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import java.util.*;

// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");

class Solution {
    static class Circle implements Comparable<Circle> {
        long right;
        long left;

        Circle(int pos, int length){
            right = (long)pos + (long)length;
            left = (long)pos - (long)length;
        }

        @Override
        public int compareTo(Circle circle) {
            if(this.left > circle.left){
                return 1;
            } else if(this.left == circle.left){
                return Long.compare(this.right, circle.right);
            } else{
                return -1;
            }
        }
    }

    public int solution(int[] A) {
        int cnt = 0;
        Circle[] circleList = new Circle[A.length];
        for (int i = 0; i < A.length; i++) {
            circleList[i] = (new Circle(i, A[i]));
        }
        Arrays.sort(circleList);

        for (int i = 0; i < A.length; i++) {
            for (int j = i+1; j < A.length; j++) {
                if(circleList[i].right >= circleList[j].left){
                    cnt++;
                }else{
                    break;
                }
            }
        }
        return cnt;
    }
}

 

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