【无标题】

快速排序和第K个数

void quickSort(int arr[], int begin, int end){
    if(begin >= end){
        return;
    }
    int pivot = arr[(begin+end)>>1];
    int i=begin-1, j=end+1; 
    //before each loop，make sure: arr[i]<pivot，arr[j]>pivot
    //that is: each arr[elem] is at the right place.
    //after each swap: retain condition.
    while(i<j){
        //before each loop, arr[i]<pivot is ok, so use do-while.
        do{
            ++i;
        }while(arr[i]<pivot);
        //when i==j, arr[i]==arr[j]>=pivot, so do-while is over.
        do{
            --j;
        }while(arr[j]>pivot);
        //so i==j is impossible 
        if(i<j){
            std::swap(arr[i], arr[j]);
        }
    }
    quickSort(arr, begin, j);
    quickSort(arr, j+1, end);
}

使用快速排序（分治算法）求解第K个数，只要对begin<=K<=end的区间进行递归排序即可。