stack(数组模拟) POJ 2559 Largest Rectangle in a Histogram

题目传送门

 /*
     题意：宽度为1，高度不等，求最大矩形面积
     stack(数组模拟)：对于每个a[i]有L[i]，R[i]坐标位置 表示a[L[i]] < a[i] < a[R[i]] 的极限情况
                   st[]里是严格单调递增，若不记录的话还要O(n)的去查找L，R，用栈的话降低复杂度
 */
 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <stack>
 #include <iostream>
 using namespace std;

 typedef long long ll;

 const int MAXN = 1e5 + ;
 const int INF = 0x3f3f3f3f;
 int a[MAXN], L[MAXN], R[MAXN];
 int st[MAXN];

 int main(void)        //POJ 2559 Largest Rectangle in a Histogram
 {
 //    freopen ("POJ_2559.in", "r", stdin);

     int n;
     while (scanf ("%d", &n) == )
     {
         if (n == )    break;
         for (int i=; i<=n; ++i)    scanf ("%d", &a[i]);
         memset (st, , sizeof (st));

         int p = ;
         for (int i=; i<=n; ++i)
         {
             while (p >=  && a[st[p-]] >= a[i])    p--;
             L[i] = (p == ) ?  : st[p-];
             st[p++] = i;
         }

         p = ;
         for (int i=n; i>=; --i)
         {
             while (p >=  && a[st[p-]] >= a[i])    p--;
             R[i] = (p == ) ? n +  : st[p-];
             st[p++] = i;
         }

         ll ans = ;
         for (int i=; i<=n; ++i)
         {
             ans = max (ans, (ll) a[i] * (R[i] - L[i] - ));
         }
         printf ("%I64d\n", ans);
     }

     return ;
 }