为什么k要加一

#include<bits/stdc++.h>
using namespace std;
const int N=5000000;
int mun[N];
int fk(int k,int l,int r){
    if(l==r)return mun[l];
    int i=l,j=r,flag=mun[(l+r)/2];
    while(i<=j){
        while(mun[i]<flag)++i;
        while(mun[j]>flag)--j;
        if(i<=j)swap(mun[i++],mun[j--]);
    }
    if(k<=j)return fk(k,l,j);
    else if(k>=i) return fk(k,i,r);
    else return fk(k,j+1,i-1);
}
int main(){
    int n,k;
    cin>>n>>k;
    for(int s=1;s<=n;++s){
        scanf("%d",&mun[s]);
    }
    fk(k,1,n);
    printf("%d",mun[k]);
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
const int N=5000000;
int mun[N];
int fk(int k,int l,int r){
    if(l==r)return mun[l];
    int i=l,j=r,flag=mun[(l+r)/2];
    while(i<=j){
        while(mun[i]<flag)++i;
        while(mun[j]>flag)--j;
        if(i<=j)swap(mun[i++],mun[j--]);
    }
    if(k<=j)return fk(k,l,j);
    else if(k>=i) return fk(k,i,r);
    else return fk(k,j+1,i-1);
}
int main(){
    int n,k;
    cin>>n>>k;
    for(int s=1;s<=n;++s){
        scanf("%d",&mun[s]);
    }
    fk(k,1,n);
    printf("%d",mun[k+1]);
    return 0;
}

为什么加一就可以过，第k个元素不就是a[k]吗？