哈希表 ---时间复杂度O(n)

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>

using namespace std;

unordered_map<int ,int > has; 

const int N = 1001001;
int res[N] , n , tag ,temp;

int main()
{
    cin >> n >> tag;
    for(int i = 0; i <= n - 1 ; i ++)
    {
        cin >> res[i];
        int t = tag - res[i];
        if(has.count(res[i])) //count是找键的
        {
            cout << has[res[i]] << " " << i;
            break;
        }
        has[t] = i;
    }

}

二分优化 ---时间复杂度O(nlogn)

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>

using namespace std;

unordered_map<int ,int > has; 

const int N = 1001001;
int res[N] , n , tag ,temp;

int main()
{
    cin >> n;
    for(int i = 0; i <= n  - 1 ; i ++) cin >> res[i];
    sort(res, res + n);
    cin >> tag;
    for(int i  = 0; i < n ; i ++) 
    {
        int t = tag - res[i];
        int l = 0 , r = n - 1;
        while(l < r)
        {
            int mid = l + r >> 1;
            if(res[mid] >= t) r = mid;
            else l = mid + 1;
        }
        if(res[r] == t){cout << i << " " << r; break;}
    }
}