hdu1506 Largest Rectangle in a Histogram

Problem Description
A histogram is a polygon composed of a sequence of rectangles aligned at a common base line. The rectangles have equal widths but may have different heights. For example, the figure on the left shows the histogram that consists of rectangles with the heights 2, 1, 4, 5, 1, 3, 3, measured in units where 1 is the width of the rectangles:

Usually, histograms are used to represent discrete distributions, e.g., the frequencies of characters in texts. Note that the order of the rectangles, i.e., their heights, is important. Calculate the area of the largest rectangle in a histogram that is aligned at the common base line, too. The figure on the right shows the largest aligned rectangle for the depicted histogram.

Input
The input contains several test cases. Each test case describes a histogram and starts with an integer n, denoting the number of rectangles it is composed of. You may assume that 1 <= n <= 100000. Then follow n integers h1, …, hn, where 0 <= hi <= 1000000000. These numbers denote the heights of the rectangles of the histogram in left-to-right order. The width of each rectangle is 1. A zero follows the input for the last test case.

Output
For each test case output on a single line the area of the largest rectangle in the specified histogram. Remember that this rectangle must be aligned at the common base line.

Sample Input

7 2 1 4 5 1 3 3
4 1000 1000 1000 1000
0

Sample Output

8
4000

    ///对于每一个a[i],用dp找出a[i]左边和右边连续大于自己的数的长度      ///l[i]表示比a[i]大的数连续的最左边的位置      ///r[i]表示比a[i]大的数连续的最右边的位置      #include<iostream>      #include<cstdlib>      #include<stdio.h>      #define LL long long      using namespace std;      const int N=100010;      LL  a[N],l[N],r[N],maxn,s;      int main()      {          int n;          while(scanf("%d",&n) && n)          {              for(int i=1;i<=n;i++)              scanf("%I64d",&a[i]);              l[1]=1;              r[n]=n;              for(int i=2;i<=n;i++)              {                  int tt=i;                  while(tt>1&&a[i]<=a[tt-1]) tt=l[tt-1];//这里是用把区间分成一段一段的,每一段都是从大到小的区间,如果这个数小于这一段的结尾,那么它必然小于这一段所有的数,再让他与前一段的结尾比较大小即可.                l[i]=tt;              }              for(int i=n-1;i>=1;i--)              {                  int tt=i;                  while(tt<n&&a[i]<=a[tt+1]) tt=r[tt+1];                  r[i]=tt;              }              maxn=0;              for(int i=1;i<=n;i++)              {                  s=(r[i]-l[i]+1)*a[i];                  if(s>maxn)                  maxn=s;              }              printf("%I64d\n",maxn);          }          return 0;      }