1049. Counting Ones (30)

The task is simple: given any positive integer N, you are supposed to count the total number of 1's in the decimal form of the integers from 1 to N. For example, given N being 12, there are five 1's in 1, 10, 11, and 12.

Input Specification:

Each input file contains one test case which gives the positive N (<=2³⁰).

Output Specification:

For each test case, print the number of 1's in one line.

Sample Input:

12

Sample Output:

5

来源： <http://www.patest.cn/contests/pat-a-practise/1049>

 

#include <string>
#include <iostream>
#include <math.h>
using namespace std;
string str;
int GetLeftNum(int a) {
	int left = 0;
	for (int i = 0; i < a; i++) {
		left *= 10;
		left += str[i] - '0';
	}
	return left;
}
int GetRightNum(int b) {
	int right = 0;
	for (int i = b+1; i < str.length(); i++) {
		right *= 10;
		right += str[i] - '0';
	}
	return right;
}
int main(void) {
	
	cin >> str;
	if (str.length() == 0) {
		cout << 1;
		return 0;
	}
	int oneCnt = 0;
	int left = 0, right = 0;
	for (int i = 0; i < str.length(); i++) {
		left = GetLeftNum(i);
		right = GetRightNum(i);
		if (str[i] - '0' > 1) {
			oneCnt += (left + 1)*pow(10, str.length() - i - 1);
		}
		else if (str[i] == '1') {
			oneCnt+= (left)*pow(10, str.length() - i - 1)+right+1;
		}
		else {
			oneCnt += (left)*pow(10, str.length() - i - 1);
		}
	}
	cout << oneCnt;
	return 0;
}

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