POJ 2559 Largest Rectangle in a Histogram

#include <stdio.h>
#include <string.h>
typedef long long LL;
const int maxn=1e5+5;
LL a[maxn];
int s[maxn];
int L[maxn];
int R[maxn];
int main() {
    int n;
    while(scanf("%d",&n),n) {
        memset(s, 0, sizeof(s));
        for (int i = 1; i <= n; i++) {
            scanf("%lld", &a[i]);
        }
        int top = 0;
        a[0] = -1;
        for (int i = 1; i <= n; i++) {
            while (top && a[i] <= a[s[top]]) {
                top--;
            }
            L[i] = s[top];
            s[++top] = i;
        }
        top = 0;
        s[top++] = n + 1;
        a[n + 1] = -1;
        for (int i = n + 1; i >= 1; i--) {
            while (top && a[i] <= a[s[top]]) {
                top--;
            }
            R[i] = s[top];
            s[++top] = i;
        }
        LL ans = 0;
        for (int i = 1; i <= n; i++) {
            LL t = (R[i] - L[i] - 1) * a[i];
            if (ans < t)
                ans = t;
        }
        printf("%lld\n", ans);
    }
    return 0;
}