#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int n;
int a[100010];
int x;
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
	
	cin>>n>>x;
	for(int i=0;i<n;i++) cin>>a[i];
	
	int cnt=0;
	int l=0,r=n-1;
	int sum;
	while(l<r){
		sum = a[l]+a[r];
		if(sum==x){
			cnt++;
			l++;
		}
		else if(sum>x){
			r--;
		}
		else{
			l++;
		}
	}
	cout<<cnt;
    return 0;
}`

Python题解

import sys
input = lambda:sys.stdin.readline().strip()

ans=0
n,x = map(int,input().split())
a = [0] + list(map(int,input().split()))

i, j = 1, n
while(i<j):
    if a[i] + a[j] == x:
        ans+=1
        i+=1
        continue
    if a[i] + a[j] < x:
        i+=1
        continue
    if a[i] + a[j] > x:
        j-=1
        continue
print(ans)

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());
		}
	}
	
}