손지민 [BOJ] 합이 0_250320

Author: jmxx219

BOJ/1000-5000번/JM_3151.java

@@ -0,0 +1,65 @@
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.StringTokenizer;
+
+public class JM_3151 {
+	static int N;
+	static int[] A;
+
+	private static int lowerBound(int startIndex, int target) {
+		int lo = startIndex, hi = N - 1;
+		int ans = N;
+		while (lo <= hi){
+			int mid = (lo + hi) / 2;
+			if(target <= A[mid]) {
+				ans = mid;
+				hi = mid - 1;
+			}
+			else lo = mid + 1;
+		}
+		return ans;
+	}
+
+	private static int upperBound(int startIndex, int target) {
+		int lo = startIndex, hi = N - 1;
+		int ans = N;
+		while (lo <= hi){
+			int mid = (lo + hi) / 2;
+			if(target < A[mid]) {
+				ans = mid;
+				hi = mid - 1;
+			}
+			else lo = mid + 1;
+		}
+		return ans;
+	}
+
+	private static long solve() {
+		Arrays.sort(A);
+		long count = 0;
+		for (int i = 0; i < N; i++) {
+			for (int j = i + 1; j < N; j++) {
+				int target = 0 - (A[i] + A[j]);
+				int lowerIndex = lowerBound(j + 1, target);
+				int upperIndex = upperBound(j + 1, target);
+				count += upperIndex - lowerIndex;
+			}
+		}
+		return count;
+	}
+
+	public static void main(String[] args) throws IOException {
+		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+		StringTokenizer st = new StringTokenizer(br.readLine());
+		N = Integer.parseInt(st.nextToken());
+
+		A = new int[N];
+		st = new StringTokenizer(br.readLine());
+		for(int i = 0; i < N; i++) {
+			A[i] = Integer.parseInt(st.nextToken());
+		}
+		System.out.println(solve());
+	}
+}