c++题解

#include<bits/stdc++.h>
using namespace std;
#define int long long
int a[100100],x,n,ans=0;
signed main(){
  cin>>n>>x;
  for(int i=1;i<=n;i++){
    cin>>a[i];
  }
  int l=1,r=n;
  while(l<r){
    if(a[l]+a[r]==x){
        ans++;
        r--;
        l++;
    }
    else if(a[l]+a[r]>x){
        r--;//如果大于x就减少右边
    }
    else l++;//否则增加左边
  }
  cout<<ans;

}

JAVA AC代码(map映射完成两数之和):

import java.util.*;
import java.util.StringTokenizer;
import java.io.*;

public class Main {

	public static void main(String[] args) {
		Map<Integer,Integer> map=new HashMap<>();
		int n=in.nextInt();
		int x=in.nextInt();
		int ans=0;
		for(int i=1;i<=n;i++){
			int number=in.nextInt();
			map.put(number,1);
			if(map.containsKey(x-number)&&number!=x-number) ans++;
		}
		out.println(ans);
		out.close();
	}
	
	

	static FastReader in=new FastReader();
	static PrintWriter out=new PrintWriter(System.out);
	static class FastReader{
		static BufferedReader br;
		static StringTokenizer st;
		FastReader(){
			br=new BufferedReader(new InputStreamReader(System.in));
		}
		String next() {
			String str="";
			while(st==null||!st.hasMoreElements()) {
				try {
					str=br.readLine();
				}catch(IOException e) {
					throw new RuntimeException(e);
				}
				st=new StringTokenizer(str);
			}
			return st.nextToken();
		}
		int nextInt() {
			return Integer.parseInt(next());
		}
		double nextDouble() {
			return Double.parseDouble(next());
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}
	
}