信息競賽進階指南--單調棧（模板）

a[n + 1] = p = 0;
for (int i = 1; i <= n + 1; i++) {
	if (a[i] > s[p]) {
		s[++p] = a[i], w[p] = 1;
	} else {
		int width=0;
		while (s[p] > a[i]) {
			width += w[p];
			ans = max(ans, (long long)width * s[p]);
			p--;
		}
		s[++p] = a[i], w[p] = width + 1;
	}
}