Largest Rectangle in a Histogram

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 3722 Accepted Submission(s): 1104

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

题目大意：给你一块柱状图，要求放一块矩形进去，使得矩形最大，求这个矩形的面积。用DP解决。

链接:http://acm.hdu.edu.cn/showproblem.php?pid=1506

代码：

#include <iostream>
#include <stdio.h>
#include <memory.h>
using namespace std;

int l[100005], r[100005];
long long h[100005];

int main()
{
    int i, n;
    long long area, MAX;            //数据很大，用long long防止溢出
    while(scanf("%d", &n), n)
    {
        for(i = 1; i <= n; i++)
        {
            scanf("%I64d", &h[i]);
            l[i] = r[i] = i;
        }
        h[0] = h[n+1] = -1; //-1防止死循环，高度可能为0
        for(i = n; i >= 1; i--)     //找右界，要从最右边开始找，不然TLE
        {
            while(h[i] <= h[ r[i]+1 ])
            {
                r[i] = r[ r[i]+1 ];
            }
        }
        for(i = 1; i <= n; i++)     //找左界，要从最左边开始找
        {
            while(h[i] <= h[ l[i]-1 ])
            {
                l[i] = l[ l[i]-1 ];
            }
        }
        MAX = 0;
        for(i = 1; i <= n; i++)
        {
            area = (r[i] - l[i] + 1) * h[i];    //面积：该点的（右界-左界+1）*高度
            MAX = max(MAX, area);
        }
        printf("%I64d\n", MAX);
    }

    return 0;
}