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15.randomizedSelect.cpp
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15.randomizedSelect.cpp
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// return ith smallest element;
// time complexity worst: n^2
#include<iostream>
#include<cstdio>
#include<cmath>
#include<ctime>
#include <cstdlib>
using namespace std;
int arr[]={13,-3,-25,20,-3,-16,-23,18,20,-7,12,-5,-22,15,-4,7};
//int arr[]={ 2, 1, 13, 5, 6, 7 };
int size = sizeof(arr)/sizeof(arr[0]);
void display(int size){
int i;
for(i=0;i<size;i++){
cout<<arr[i]<<" ";
}
cout<<"\n";
}
void swap(int *a, int *b){
int temp;
temp=*a;
*a=*b;
*b=temp;
}
int partition(int low, int high){
int item = arr[high];
int i = low - 1;
for(int j = low;j<high;j++){
if(arr[j]<=item){
i += 1;
swap(arr[i],arr[j]);
}
}
swap(arr[i+1],arr[high]);
return i+1;
}
int random(int low, int high){
srand(time(NULL));
return low+rand()%(high-low);
}
int randomizedPartition(int low, int high){
int i=random(low,high);
swap(arr[high],arr[i]);
return partition(low,high);
}
int randomizedSelect(int low, int high, int i_th){
if(low==high){
return arr[low];
}
int mid = randomizedPartition(low,high);
int k = mid - low + 1;
if(i_th==k){
//pivot is answer
return arr[mid];
}
else if(i_th<k){
return randomizedSelect(low,mid-1,i_th);
}
else{
return randomizedSelect(mid+1,high,i_th-k);
}
}
int main(){
int low = 0;
int high = size-1;
int i_th;
display(size);
cout<<"i_th smallest: ";
cin>>i_th;
int found=randomizedSelect(low,high,i_th);
cout<<found;
return 0;
}