关于如何将 sort 换成归并排序

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

using ld = long double;
using ll = long long;
typedef struct Point { ld x; ld y; } Pt;
const int maxn = 2e5+5;
const ld inf = 2 << 20;

bool cmp(const Pt &a, const Pt &b){
	if(a.x==b.x) return a.y<b.y;
	else return a.x<b.x;
}

ld dist(Pt i, Pt j) 
{
    ld x = (i.x - j.x) * (i.x - j.x);
	ld y = (i.y - j.y) * (i.y - j.y);
    return sqrt(x + y);
}

Pt p[maxn];
int tmp[maxn];
int n;

ld merge(int l, int r){
	if(l==r) return inf;
	else if(l+1==r) return dist(p[l],p[r]);
	int mid = l+r>>1;
	ld d1 = merge(l,mid), d2 = merge(mid+1,r);
	ld d = min(d1,d2);
	int tp=0; 
	for(int i = l; i<=r; i++){
		if(fabs(p[mid].x-p[i].x) < d) tmp[++tp] = i;
	}
	stable_sort(tmp+1,tmp+1+tp,[](int a, int b){ return p[a].y < p[b].y; }); // sort here !!!
	for(int i = 1; i<=tp; i++){
		for(int j = i+1; (j<=tp)&&(p[tmp[j]].y-p[tmp[i]].y < d); j++){
			ld d3 = dist(p[tmp[j]],p[tmp[i]]);
			d=min(d,d3);
		}
	}
	return d;
}

int main(){
	cin>>n;
	for(int i = 1; i<=n; i++) cin>>p[i].x>>p[i].y;
	stable_sort(p+1,p+1+n,cmp);
	return printf("%.4Lf",merge(1,n)), 0;
}