int maxsofar = 0;
for (int i = 0; i < n; i++) {
    for (int j = i; j < n; j++) {
        sum = 0;
        for (int k = i; k <= j; k++) {
            sum += a[k];
        }
        maxsofar = max(maxsofar, sum);
    }
}

int maxsofar = 0;
for (int i = 0; i < n; i++) {
    int sum = 0;
    for (int j = i; j < n; j++) {
        sum += a[j];
        maxsofar = max(maxsofar, sum);
    }
}

int maxsofar = 0;
cumarr[-1] = 0;           // cumarr 计算从 a[0] 到 a[i] 的和
for (int i = 0; i < n; i++) {
    cumarr[i] = a[i] + cumarr[i-1];
}

int sum;
for (int i = 0; i < n; i++) {
    for (int j = i; j < n; j++) {
        sum = cumarr[j] - cumarr[i-1];
        maxsofar = max(maxsofar, sum);
    }
}

if (l > r) {
        return 0;
    }
    if (l == r) {
        return a[l];
    }
    int mid = (l+r) / 2;

    int lmax = 0;
    int lsum = 0;
    for (int i = mid; i >= l; i--) {
        lsum += a[i];
        lmax = max(lsum, lmax);
    }

    int rmax = 0;
    int rsum = 0;
    for (int i = mid+1; i <= r; i++) {
        rsum += a[i];
        rmax = max(rsum, rmax);
    }
    return max(lmax+rmax, max(maxsum(l, mid), maxsum(mid+1, r)));
}

int maxsofar = 0;
int maxendinghere = 0;
for (int i = 0; i < n; i++) {
    maxendinghere = max(maxendinghere, 0);
    maxsofar = max(maxsofar, maxendinghere);
}