給定一個十進位制正整數N，寫下從1開始，到N的所有正整數，然後數一下其中出現的所有“1”的個數。例如：N =12，我們會寫下1,2,3,4,5,6,7,8,9,10,11,12。這樣1出現的個數為5

java程式碼實現

public class demo1 {

	public static void main(String[] args) {
		Sum sum = new Sum();
		System.out.println("請輸入一個正整數N：");
		int v = new Scanner(System.in).nextInt();
		System.out.println("從1開始，到N的所有整數，1出現的個數：" + sum.sum1(v));
	}
}
class Sum{
	public int sum1(int n){
		int count = 0;
		int factor =1;
		int lowerNum = 0;
		int currNum = 0;
		int higherNum = 0;
		while(n/factor !=0){
			lowerNum = n-(n/factor)*factor;
			currNum = (n/factor)%10;
			higherNum = n/(factor*10);
			switch(currNum){
			case 0:
				count +=higherNum*factor;
				break;
			case 1:
				count +=higherNum*factor+lowerNum+1;
				break;
			default:
				count +=(higherNum+1)*factor;
				break;
			}
			factor *=10;
		}
		return count;
	}
}