#436. 子串的最大差

http://oj.daimayuan.top/problem/436

使用两次单调栈

#include<bits/stdc++.h>
using namespace std;
const int N = 1e6;
class Node {
	public:
		long long int val,num;
		int lma,rma;
		int lmi,rmi;
};
Node a[N];
long long subArrayRanges(int n) {
	stack<Node>icrs,dcrs; //升降
	icrs.push(a[1]);
	dcrs.push(a[1]);
	for(int i = 2 ; i <= n ; i++) {
		while(icrs.size() && icrs.top().val <= a[i].val ) { //升
			int j = icrs.top().num;
			a[j].rma = i;
			icrs.pop();
		}
		if(icrs.size())
			a[i].lma = icrs.top().num;
		else a[i].lma = 0;
		icrs.push(a[i]);

		while(dcrs.size() && dcrs.top().val >= a[i].val ) { //降
			int j = dcrs.top().num;
			a[j].rmi = i;
			dcrs.pop();
		}
		if(dcrs.size())
			a[i].lmi = dcrs.top().num;
		else a[i].lmi = 0;
		dcrs.push(a[i]);
	}
	while(icrs.size()) {
		int j = icrs.top().num;
		icrs.pop();
		a[j].rma = n+1;
		icrs.size();
	}
	while(dcrs.size()) {
		int j = dcrs.top().num;
		dcrs.pop();
		a[j].rmi = n+1;
		dcrs.size();
	}
	long long sum = 0;
	for(int i = 1 ; i <= n ; i++) {
		sum += a[i].val*(a[i].rma - i)*(i - a[i].lma);
		sum -= a[i].val*(a[i].rmi - i)*(i - a[i].lmi);
	}
	return sum;
}
int main() {
	int n;
	cin >> n;
	for(int i = 1 ; i <= n ; i++) {
		cin >> a[i].val;
		a[i].num = i;
	}
	cout << subArrayRanges(n);
}