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| 1 | +import java.io.BufferedReader; |
| 2 | +import java.io.IOException; |
| 3 | +import java.io.InputStreamReader; |
| 4 | +import java.util.Arrays; |
| 5 | +import java.util.StringTokenizer; |
| 6 | + |
| 7 | +public class JM_3151 { |
| 8 | + static int N; |
| 9 | + static int[] A; |
| 10 | + |
| 11 | + private static int lowerBound(int startIndex, int target) { |
| 12 | + int lo = startIndex, hi = N - 1; |
| 13 | + int ans = N; |
| 14 | + while (lo <= hi){ |
| 15 | + int mid = (lo + hi) / 2; |
| 16 | + if(target <= A[mid]) { |
| 17 | + ans = mid; |
| 18 | + hi = mid - 1; |
| 19 | + } |
| 20 | + else lo = mid + 1; |
| 21 | + } |
| 22 | + return ans; |
| 23 | + } |
| 24 | + |
| 25 | + private static int upperBound(int startIndex, int target) { |
| 26 | + int lo = startIndex, hi = N - 1; |
| 27 | + int ans = N; |
| 28 | + while (lo <= hi){ |
| 29 | + int mid = (lo + hi) / 2; |
| 30 | + if(target < A[mid]) { |
| 31 | + ans = mid; |
| 32 | + hi = mid - 1; |
| 33 | + } |
| 34 | + else lo = mid + 1; |
| 35 | + } |
| 36 | + return ans; |
| 37 | + } |
| 38 | + |
| 39 | + private static long solve() { |
| 40 | + Arrays.sort(A); |
| 41 | + long count = 0; |
| 42 | + for (int i = 0; i < N; i++) { |
| 43 | + for (int j = i + 1; j < N; j++) { |
| 44 | + int target = 0 - (A[i] + A[j]); |
| 45 | + int lowerIndex = lowerBound(j + 1, target); |
| 46 | + int upperIndex = upperBound(j + 1, target); |
| 47 | + count += upperIndex - lowerIndex; |
| 48 | + } |
| 49 | + } |
| 50 | + return count; |
| 51 | + } |
| 52 | + |
| 53 | + public static void main(String[] args) throws IOException { |
| 54 | + BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); |
| 55 | + StringTokenizer st = new StringTokenizer(br.readLine()); |
| 56 | + N = Integer.parseInt(st.nextToken()); |
| 57 | + |
| 58 | + A = new int[N]; |
| 59 | + st = new StringTokenizer(br.readLine()); |
| 60 | + for(int i = 0; i < N; i++) { |
| 61 | + A[i] = Integer.parseInt(st.nextToken()); |
| 62 | + } |
| 63 | + System.out.println(solve()); |
| 64 | + } |
| 65 | +} |
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