Hdu 1506 Largest Rectangle in a Histogram （DP求最大长方形面积）

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

思路：用dp思想求得每一列能往左，右延伸的最长长度。不能一个一个遍历，学会处理后一个数据时用上 上一个已更新的数据

注意：用__int64 %I64 输入输出（可能TLE）

代码：

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define LL __int64
#define maxn 100000+5
LL h[maxn];
int l[maxn],r[maxn];
int main(){
int n;
while(scanf("%d",&n)!=EOF && n){
h[0] = h[n+1] = -1;
for (int i = 1; i <= n; i++){
scanf("%I64d",&h[i]);
l[i] = r[i] = i;
}
for (int i = 1; i <= n; i++){
while( l[i] >= 1 && h[ l[i]-1 ] >= h[i]){
l[i] = l[ l[i]-1 ];
}
}
for (int i = n; i >= 1; i--)
{
while( r[i] <= n && h[ r[i]+1 ] >= h[i]){
r[i] = r[ r[i]+1 ];
}
}
LL ans = 0,tmp = 0;
for (int i = 1; i <= n; i++)
{
tmp = (r[i]-l[i]+1)*h[i];
//ans = max(ans,tmp);
if(ans < tmp)ans = tmp;
}
printf("%I64d\n",ans);
}
return 0;
}