Looking for 1 in the series

I. Topics and Requirements

• Given a positive integer in decimal, write down all integers starting with 1, to N, and then count the number of "1" that appears.

• Requirement: Write a function f (n) to return the number of "1" occurrences between 1 and N. For example F (12) = 5.

Within a 32-bit integer range, the maximum n of the "F (n) =n" that satisfies the condition is what.

Second, design ideas

The number of 2-digit and 3-digit 1 is calculated first, and then the algorithm of 1 is recursively programmed.

Third, the experimental code

#include <iostream.h>intMain () {intnum; inttag=1; intCount=0; intHeigher,cur,lower; cout<<"Please enter data:"; CIN>>num;  while(num/tag!=0) {Heigher=num/(tag*Ten); Cur= (Num/tag)%Ten; Lower=num%tag; Switch(cur) { Case 0: Count=count+heigher*tag;  Break;  Case 1: Count=count+heigher*tag+lower+1;  Break; default: Count=count+ (heigher+1)*tag;  Break; } tag=tag*Ten; } cout<<"1 The number of occurrences is:"<<count<<Endl; return 0;}

Four, the experiment:

Looking for 1 in the series