【BZOJ 4582】【Usaco2016 Open】Diamond Collector

http://www.lydsy.com/JudgeOnline/problem.php?id=4582
排好序后用两个指针直接$O(n)$扫，貌似这个东西学名"two pointers"？

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

const int N = 50003;

int pre[N], aft[N], a[N], n, k, cnt;

int main() {
	scanf("%d %d", &n, &k);
	for (int i = 1; i <= n; ++i)	
		scanf("%d", a + i);
	stable_sort(a + 1, a + n + 1);
	
	int tail = 1, head = n;
	for (int i = 1; i <= n; ++i) {
		while (tail <= n && a[tail] - a[i] <= k) ++tail;
		aft[i] = tail - i;
	}
	
	for (int i = n; i >= 1; --i) {
		while (head >= 1 && a[i] - a[head] <= k) --head;
		pre[i] = i - head;
	}
	
	for (int i = 2; i <= n; ++i)
		pre[i] = max(pre[i], pre[i - 1]);
	for (int i = n - 1; i >= 1; --i)
		aft[i] = max(aft[i], aft[i + 1]);
	
	int ans = 0;
	for (int i = 2; i <= n; ++i)
		ans = max(ans, pre[i - 1] + aft[i]);
	printf("%d\n", ans);	
	return 0;
}