[hdu1506]单调队列（栈）

题意：http://acm.hdu.edu.cn/showproblem.php?pid=1506看图一目了然。两个方向单调队列维护下。

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <cstdlib>
 #include <cstring>
 #include <map>
 #include <queue>
 #include <deque>
 #include <cmath>
 #include <vector>
 #include <ctime>
 #include <cctype>
 #include <set>

 using namespace std;

 #define mem0(a) memset(a, 0, sizeof(a))
 #define lson l, m, rt << 1
 #define rson m + 1, r, rt << 1 | 1
 #define define_m int m = (l + r) >> 1
 #define Rep(a, b) for(int a = 0; a < b; a++)
 #define lowbit(x) ((x) & (-(x)))
 #define constructInt4(name, a, b, c, d) name(int a = 0, int b = 0, int c = 0, int d = 0): a(a), b(b), c(c), d(d) {}
 #define constructInt3(name, a, b, c) name(int a = 0, int b = 0, int c = 0): a(a), b(b), c(c) {}
 #define constructInt2(name, a, b) name(int a = 0, int b = 0): a(a), b(b) {}

 typedef double db;
 typedef long long LL;
 typedef pair<int, int> pii;
 typedef multiset<int> msi;
 typedef multiset<int>::iterator msii;
 typedef set<int> si;
 typedef set<int>::iterator sii;

 const int dx[] = {, , -, , , , -, -};
 const int dy[] = {, -, , , -, , , -};
 const int maxn = 1e6 + ;
 const int maxm = 1e5 + ;
 const int MD = 1e9 +;
 const int INF = 1e9 + ;

 template<class T> struct MonotoneQueue{
     deque<T> Q;
     MonotoneQueue<T>() { Q.clear(); }
     void clear() { Q.clear(); }
     bool empty() { return Q.empty(); }
     void add_back(T x) { while (!Q.empty() && !(Q.back() < x)) Q.pop_back(); Q.push_back(x); }
     void pop_front() { Q.pop_front(); }
     T back2() { return *(Q.end() - ); }
     T front() { return Q.front(); }
 };

 struct pair1 {
     int val, pos;
     bool operator < (const pair1 &a) const {
         return val < a.val;
     }
     constructInt2(pair1, val, pos);
 };

 MonotoneQueue<pair1> UQ;

 int a[], L[];

 int main() {
     //freopen("in.txt", "r", stdin);
     int n;
     while (cin >> n, n) {
         UQ.clear();
         UQ.add_back(pair1(-, -));
         for (int i = ; i < n; i++) {
             scanf("%d", a + i);
             UQ.add_back(pair1(a[i], i));
             L[i] = UQ.back2().pos + ;
         }
         UQ.clear();
         UQ.add_back(pair1(-, n));
         LL ans = ;
         for (int i = n - ; i >= ; i--) {
             UQ.add_back(pair1(a[i], i));
             ans = max(ans, (LL)a[i] * (UQ.back2().pos - L[i]));
         }
         cout << ans << endl;
     }
     return ;
 }