unordered_map

#include <iostream>
#include <unordered_map>
using namespace std;
const int N = 1E5 + 10;
int a[N];
unordered_map<int, int> mp;
int main(){
    int n, m, x;    cin >> n >> m >> x;
    for(int i = 0; i < n; i ++ ){
        int tmp;    scanf("%d", &tmp);
        mp.insert({tmp, i});
    }
    for(int i = 0; i < m; i ++ )    scanf("%d", &a[i]);
    for(int i = 0; i < m; i ++ ){
        if(mp.find(x - a[i]) != mp.end()){
            cout << mp[x - a[i]] << ' ' << i << endl;
            break;
        }
    }
    return 0;
}

双指针（输入数据有序）

#include <iostream>
#include <cstdio>
using namespace std;
const int N = 1E5 + 10;
int a[N], b[N];
int main(){
    int n, m, x;    cin >> n >> m >> x;
    for(int i = 0; i < n; i ++ )    scanf("%d", &a[i]);
    for(int i = 0; i < m; i ++ )    scanf("%d", &b[i]);
    int i = 0, j = m - 1;
    while(i < n && j >= 0){
        int sum = a[i] + b[j]; 
        if(sum == x)    break;
        else if(sum < x)    i ++ ;
        else    j -- ;
    }
    cout << i << ' ' << j << endl;
    return 0;
}