leetcode-327. 区间和的个数-C语言

/*
* 算法思想
* 使用归并排序的算法思想
* 1. 使用sums[i]数组放0->i-1的累加和，sums[0] = 0;
* 2. 对于任意的i!=j,  lower <= sums[i]-sums[j] <= upper满足条件,则为一种可能的序列区间；
* 3. 对于有序的数组，取符合条件的返回，可使用双指针的方法。
*
*/

void print(long long *arr, int len){
    int i=0;
    for(i=0; i<len; i++){
        printf("%d,  ", arr[i]);
    }
        
    printf("\n");
}
/*
 * merge two ordered arrays
 */
void merge(long long *sums, int start, int mid, int end){
    long long buf[end-start+1];
    int index=0, i=start, j=mid+1;
    while(i<=mid && j<=end){
        if(sums[i] < sums[j]){
            buf[index++] = sums[i++];
        }else{
            buf[index++] = sums[j++];
        }
    }
    
    while(i<=mid)
        buf[index++] = sums[i++];
    
    while(j<=end)
        buf[index++] = sums[j++];
    
    memcpy(sums+start, buf, sizeof(long long) * (index));
}

int merge_sort(long long *sums, int start, int end, long long l, long long u){
    int mid, ret=0, i, m, n, ret1, ret2=0;
    
    if(end - start < 1) return 0;
    
    mid = (start + end) / 2;
    m = mid+1;
    n = mid+1;
    
    /* sparate array to two parts, first part : [start, mid], second part: [mid+1, end]
     * deal with [start, mid] , [mid+1, end] sparately  */
    ret1 = merge_sort(sums, start, mid, l, u) + merge_sort(sums, mid+1, end, l, u);
    
    /* deal with current situation before merge operation;
     * Notes:
     * 1. on this condition, arr[start, mid] and  arr[mid+1, end] are ordered increasingly;
     * 2. 
     */
    for(i=start; i<=mid; i++){
        while(m <= end && sums[m]-sums[i] < l)
            m++;
        while(n <= end && sums[n]-sums[i] <= u)
            n++;
        ret2 += (n-m);
    }
    
    merge(sums, start, mid, end);
    
    ret = ret1 +ret2;
    
    return ret;
}


int countRangeSum(int* ns, int len, int lower, int upper){
    long long i, j, cnt=0;
    if(!ns || !len) return 0;
    long long sums[len+1], ret=0;
    long long tmp;
    
    sums[0] = 0;
    for(i=0; i<len; i++){
        cnt += ns[i];
        sums[i+1] = cnt;
    }
 
    ret = merge_sort(sums, 0, len, lower, upper);

    return ret ;
}