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Posted 2021-01-12 saramoring

int partition2(int[] arr,int Begin,int End,int Pivot)
{
    int i = Begin;
    int j = End;
    while(i<j)
    {
        while( i<j && arr[j]>=Pivot)--j;
        while(i<j && arr[i]<= Pivot)++i;
        swap(arr[i],arr[j]);
    }
    //?÷è·partition±???￡??úioíi×ó±?μ?êy??D?óúμèóúPivot
    if(arr[i]>Pivot)
        i = i-1;
    return i;
}
int Select(int[] arr,int Begin,int End, int k)
{
    if(End-Begin+1<75)
    {
        sort(arr+Begin,arr+End+1);
        return arr[Begin+k-1];
    }
    int count = End-Begin+1;
    int G5 = floor(count/5);
    for(int i =0;i<G5;++i)
    {
        sort(arr+Begin+5*i,arr+Begin+5*i+5);
        swap(arr[Begin+i],arr[Begin+5*i+2]);
    }
    
    //?ò?D??êy?Dμ??D??êy
    int x = Select(arr,Begin,Begin+G5-1,G5/2);
    int pos_x = partition2(arr, Begin, End ,x);
    int len = pos_x-Begin+1;
    if(k<=len)
    {
        return Select(arr,Begin,pos_x,k);
    }
    else
    {
        return Select(arr,pos_x+1,End,k-len);
    }
}