Added C++ solution to kth Largest Element in O(n)

The previous solution was in O(nlogn), added a more efficient O(n) solution with randomized quickselect

search/kth_largest_in_array/Java/Solution_cpp_O(n).cpp

@@ -0,0 +1,37 @@
+#include<bits/stdc++.h>
+#define ll long long
+using namespace std;
+
+ll partition(ll a[],int l,int r){
+    int k=l+rand()%(r-l+1),i=l;
+    swap(a[l],a[k]);
+    ll key=a[l];
+    for(int j=l+1;j<=r;j++){
+        if(a[j]<=key){
+            i++;
+            swap(a[i],a[j]);
+        }
+    }
+    swap(a[l],a[i]);
+    return i;
+}
+
+ll kth_largest(ll a[],ll l,ll r,ll k){
+    if(k>r-l+1||k<=0) return LLONG_MAX;
+    int p=partition(a,l,r);
+    if(p==r-k+1) return a[p];
+    if(p>r-k+1) return kth_largest(a,l,p-1,k-r+p-1);
+    return kth_largest(a,p+1,r,k);
+}
+
+int main(){
+    int n,k;
+    cin>>n>>k;
+    srand(time_t(NULL));
+    ll a[n];
+    for(int i=0;i<n;i++) cin>>a[i];
+    ll ans=kth_largest(a,0,n-1,k);
+    if(ans==LLONG_MAX) cout<<"No Kth largest element\n";
+    else cout<<ans<<'\n';
+    return 0;
+}