Merge Intervals

       Given a collection of intervals, merge all overlapping intervals.

       For example,
Given [1,3], [2,6], [8,10], [15,18],
return [1,6], [8,10], [15,18].

 

       思路：本题的难度虽然是hard，但是实际上比较简单，就是先排序，后合并。下面的代码使用了realloc（584 ms），如果直接使用malloc分配足够的内存，结果竟然会变慢（692 ms），呵呵。代码如下：

struct Interval
{
    int start;
    int end;
};
 
int compar(constvoid *a, const void *b)
{
    return  ((struct Interval *)a)->start- ((struct Interval *)b)->start;
}
 
struct Interval*merge(struct Interval* intervals, int intervalsSize,int* returnSize)
{
    int i;
    int laststart, lastend;
    struct Interval *res = NULL;
    int reslen = 0;
 
    if(intervalsSize == 0)  return res;
    qsort(intervals, intervalsSize,sizeof(struct Interval), compar);
 
    laststart = intervals[0].start;
    lastend = intervals[0].end;
   
    for(i = 1; i <intervalsSize; i++)
    {
        if(intervals[i].start<= lastend)
        {
            if(lastend < intervals[i].end)
            {
                lastend = intervals[i].end;
            }
        }
        else
        {
            reslen++;
            res = realloc(res,reslen*sizeof(struct Interval));
            res[reslen-1].start= laststart;
            res[reslen-1].end= lastend;
 
            laststart = intervals[i].start;
            lastend = intervals[i].end;
        }
    }
   
    reslen++;
    res = realloc(res, reslen*sizeof(structInterval));
    res[reslen-1].start =laststart;
    res[reslen-1].end =lastend;
 
    *returnSize = reslen;
    return res;
}