No of 1's in <=n
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#include <iostream>
using namespace std;
int count1(int n){
if(n == 0){
return 0;
}
if(n == 1){
return 1;
}
int countof1innm1 = count1(n-1);
int countinn = 0;
while(n !=0){
int rem = n%10;
if(rem ==1){
countinn++;
}
n = n/10;
}
return countinn + countof1innm1;
}
int main() {
int n;
cin>>n;
cout<<count1(n);
}
Cpp
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