【HDOJ】1506 Largest Rectangle in a Histogram

Twitter还是Amazon拿这个题目当过面试题。DP区间求面积。

/* 1506 */
#include <cstdio>
#include <cstring>
#include <cstdlib>

#define MAXN 100005
#define INF 999999

int l[MAXN], r[MAXN];
__int64 a[MAXN];

__int64 max(__int64 a, __int64 b) {
    return a>b ? a:b;
}

int main() {
    int n;
    int i, j, k;
    __int64 ans;

    #ifndef ONLINE_JUDGE
        freopen("data.in", "r", stdin);
    #endif

    while (scanf("%d",&n)!=EOF && n) {
        for (i=1; i<=n; ++i)
            scanf("%I64d", &a[i]);
        a[0] = a[n+1] = -INF;
        l[0] = 0;
        for (i=1; i<=n; ++i) {
            if (a[i-1] < a[i]) {
                l[i] = i;
            } else {
                j = i;
                while (j--) {
                    if (a[j] < a[i]) {
                        l[i] = j+1;
                        break;
                    } else {
                        j = l[j];
                    }
                }
            }
        }
        r[n+1] = n+1;
        for (i=n; i>0; --i) {
            if (a[i+1] < a[i]) {
                r[i] = i;
            } else {
                j = i;
                while (j++ <= n) {
                    if (a[j] < a[i]) {
                        r[i] = j - 1;
                        break;
                    } else {
                        j = r[j];
                    }
                }
            }
        }

        ans = -INF;
        for (i=1; i<=n; ++i) {
            ans = max(ans, a[i]*(r[i]-l[i]+1));
        }
        printf("%I64d\n", ans);
    }

    return 0;
}