Pairs having sum equal to target, C++ (gcc)

Pairs having sum equal to target

#include<bits/stdc++.h>
using namespace std;

#define F first
#define S second

int main(){
	int t;
	cin >> t;
	while(t--){
	    int n, target;
	    cin >> n >> target;
	    vector<int> v(n);
	    for(int j = 0; j < n; j++) cin >> v[j];
	    sort(v.begin(), v.end());
	    
	    int ans = 0;
	    int l_low = 0, l_high = 0, r_low = n-1, r_high = n-1;
	    while(l_high <= r_low){
	        while(l_high < n && v[l_high] == v[l_low]) l_high ++;
	        while(r_low >= 0 && v[r_low] == v[r_high]) r_low --;
	        
	        if(v[l_low] + v[r_high] == target){
	            int cnt_l = l_high - l_low;
	            int cnt_r = r_high - r_low;
	            
	            if(v[l_low] == v[r_high]) ans += cnt_l*(cnt_l-1)/2;
	            else ans += cnt_l*cnt_r;
	            
	            l_low = l_high, r_high = r_low;
	            continue;
	        }
	        
	        if(v[l_low] + v[r_high] < target) {
	            l_low = l_high;
	            r_low ++;
	            continue;
	        }
	        
	        if(v[l_low] + v[r_high] > target) {
	            r_high = r_low;
	            l_high --;
	            continue;
	        }
	        
	        assert(false);
	    }
	    cout << ans << '\n';
	}
	return 0;
}


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